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| author | Guido van Rossum <guido@python.org> | 1997-08-18 19:55:01 (GMT) |
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| committer | Guido van Rossum <guido@python.org> | 1997-08-18 19:55:01 (GMT) |
| commit | 6688d35c4378327e11e351f2536621198b0320d0 (patch) | |
| tree | 4196bc40b4d239abb2713fc81210d91604bbbc5f /Misc | |
| parent | 25f93036bbaba0371bb64acfd91a8a515b77f6a2 (diff) | |
| download | cpython-6688d35c4378327e11e351f2536621198b0320d0.zip cpython-6688d35c4378327e11e351f2536621198b0320d0.tar.gz cpython-6688d35c4378327e11e351f2536621198b0320d0.tar.bz2 | |
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diff --git a/Misc/FAQ b/Misc/FAQ deleted file mode 100644 index 57c1737..0000000 --- a/Misc/FAQ +++ /dev/null @@ -1,2272 +0,0 @@ -Subject: FAQ: Python -- an object-oriented language -Newsgroups: comp.lang.python,comp.answers,news.answers -Followup-to: comp.lang.python -From: guido@cnri.reston.va.us (Guido van Rossum) -Reply-to: guido@cnri.reston.va.us (Guido van Rossum) -Expires: Sun, 1 Dec 1996 00:00:00 GMT -Supersedes: <DxJ3t1.CJv@cwi.nl> -Approved: news-answers-request@MIT.Edu - -Archive-name: python-faq/part1 -Submitted-by: Guido van Rossum <guido@cnri.reston.va.us> -Version: $Revision$ -Last-modified: $Date$ - -This article contains answers to Frequently Asked Questions about -Python (an object-oriented interpreted programming language -- see -the answer to question 1.1 for a short overview). - -Copyright 1993-1996 Guido van Rossum. Unchanged electronic -redistribution of this FAQ is allowed. Printed redistribution only -with permission of the author. No warranties. - -Author's address: - Guido van Rossum - C.N.R.I. - 1895 Preston White Drive - Reston, VA 20191 - U.S.A. -Email: <guido@python.org>, <guido@cnri.reston.va.us> - -The latest version of this FAQ is available by anonymous ftp from -<URL:ftp://ftp.python.org/pub/python/doc/FAQ>. It will also be posted -regularly to the newsgroups comp.answers <URL:news:comp.answers> and -comp.lang.python <URL:news:comp.lang.python>. - -Many FAQs, including this one, are available by anonymous ftp -<URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/>. The name under -which a FAQ is archived appears in the Archive-name line at the top of -the article. This FAQ is archived as python-faq/part1 -<URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/python-faq/part1>. - -There's a mail server on that machine which will send you files from -the archive by e-mail if you have no ftp access. You send a e-mail -message to <mail-server@rtfm.mit.edu> containing the single word help -in the message body to receive instructions. - -This FAQ is divided in the following chapters: - - 1. General information and availability - 2. Python in the real world - 3. Building Python and Other Known Bugs - 4. Programming in Python - 5. Extending Python - 6. Python's design - 7. Using Python on non-UNIX platforms - -To find the start of a particular chapter, search for the chapter number -followed by a dot and a space at the beginning of a line (e.g. to -find chapter 4 in vi, type /^4\. /). - -Here's an overview of the questions per chapter: - - 1. General information and availability - 1.1. Q. What is Python? - 1.2. Q. Why is it called Python? - 1.3. Q. How do I obtain a copy of the Python source? - 1.4. Q. How do I get documentation on Python? - 1.5. Q. Are there other ftp sites that mirror the Python distribution? - 1.6. Q. Is there a newsgroup or mailing list devoted to Python? - 1.7. Q. Is there a WWW page devoted to Python? - 1.8. Q. Is the Python documentation available on the WWW? - 1.9. Q. Is there a book on Python, or will there be one out soon? - 1.10. Q. Are there any published articles about Python that I can quote? - 1.11. Q. Are there short introductory papers or talks on Python? - 1.12. Q. How does the Python version numbering scheme work? - 1.13. Q. How do I get a beta test version of Python? - 1.14. Q. Are there copyright restrictions on the use of Python? - 1.15. Q. Why was Python created in the first place? - - 2. Python in the real world - 2.1. Q. How many people are using Python? - 2.2. Q. Have any significant projects been done in Python? - 2.3. Q. Are there any commercial projects going on using Python? - 2.4. Q. How stable is Python? - 2.5. Q. What new developments are expected for Python in the future? - 2.6. Q. Is it reasonable to propose incompatible changes to Python? - 2.7. Q. What is the future of Python? - 2.8. Q. What is the PSA, anyway? - 2.9. Q. How do I join the PSA? - 2.10. Q. What are the benefits of joining the PSA? - - 3. Building Python and Other Known Bugs - 3.1. Q. Is there a test set? - 3.2. Q. When running the test set, I get complaints about floating point - operations, but when playing with floating point operations I cannot - find anything wrong with them. - 3.3. Q. Link errors after rerunning the configure script. - 3.4. Q. The python interpreter complains about options passed to a - script (after the script name). - 3.5. Q. When building on the SGI, make tries to run python to create - glmodule.c, but python hasn't been built or installed yet. - 3.6. Q. I use VPATH but some targets are built in the source directory. - 3.7. Q. Trouble building or linking with the GNU readline library. - 3.8. Q. Trouble with socket I/O on older Linux 1.x versions. - 3.9. Q. Trouble with prototypes on Ultrix. - 3.10. Q. Other trouble building Python on platform X. - 3.11. Q. How to configure dynamic loading on Linux. - 3.12. Q: I can't get shared modules to work on Linux 2.0 (Slackware96)? - 3.13. Q: Trouble when making modules shared on Linux. - 3.14. Q. How to use threads on Linux. - 3.15. Q. Errors when linking with a shared library containing C++ code. - 3.16. Q. I built with tkintermodule.c enabled but get "Tkinter not found". - 3.17. Q. I built with Tk 4.0 but Tkinter complains about the Tk version. - 3.18. Q. Link errors for Tcl/Tk symbols when linking with Tcl/Tk. - 3.19. Q. I configured and built Python for Tcl/Tk but "import Tkinter" - fails. - 3.20. Q. Tk doesn't work right on DEC Alpha. - 3.21. Q. Several common system calls are missing from the posix module. - 3.22. Q. ImportError: No module named string, on MS Windows. - 3.23. Q. Core dump on SGI when using the gl module. - - 4. Programming in Python - 4.1. Q. Is there a source code level debugger with breakpoints, step, - etc.? - 4.2. Q. Can I create an object class with some methods implemented in - C and others in Python (e.g. through inheritance)? (Also phrased as: - Can I use a built-in type as base class?) - 4.3. Q. Is there a curses/termcap package for Python? - 4.4. Q. Is there an equivalent to C's onexit() in Python? - 4.5. Q. When I define a function nested inside another function, the - nested function seemingly can't access the local variables of the - outer function. What is going on? How do I pass local data to a - nested function? - 4.6. Q. How do I iterate over a sequence in reverse order? - 4.7. Q. My program is too slow. How do I speed it up? - 4.8. Q. When I have imported a module, then edit it, and import it - again (into the same Python process), the changes don't seem to take - place. What is going on? - 4.9. Q. How do I find the current module name? - 4.10. Q. I have a module in which I want to execute some extra code - when it is run as a script. How do I find out whether I am running as - a script? - 4.11. Q. I try to run a program from the Demo directory but it fails - with ImportError: No module named ...; what gives? - 4.12. Q. I have successfully built Python with STDWIN but it can't - find some modules (e.g. stdwinevents). - 4.13. Q. What GUI toolkits exist for Python? - 4.14. Q. Are there any interfaces to database packages in Python? - 4.15. Q. Is it possible to write obfuscated one-liners in Python? - 4.16. Q. Is there an equivalent of C's "?:" ternary operator? - 4.17. Q. My class defines __del__ but it is not called when I delete the - object. - 4.18. Q. How do I change the shell environment for programs called - using os.popen() or os.system()? Changing os.environ doesn't work. - 4.19. Q. What is a class? - 4.20. Q. What is a method? - 4.21. Q. What is self? - 4.22. Q. What is a unbound method? - 4.23. Q. How do I call a method defined in a base class from a derived - class that overrides it? - 4.24. Q. How do I call a method from a base class without using the - name of the base class? - 4.25. Q. How can I organize my code to make it easier to change the base - class? - 4.26. Q. How can I find the methods or attributes of an object? - 4.27. Q. I can't seem to use os.read() on a pipe created with os.popen(). - 4.28. Q. How can I create a stand-alone binary from a Python script? - 4.29. Q. What WWW tools are there for Python? - 4.30. Q. How do I run a subprocess with pipes connected to both input - and output? - 4.31. Q. How do I call a function if I have the arguments in a tuple? - 4.32. Q. How do I enable font-lock-mode for Python in Emacs? - 4.33. Q. Is there an inverse to the format operator (a la C's scanf())? - 4.34. Q. Can I have Tk events handled while waiting for I/O? - 4.35. Q. How do I write a function with output parameters (call by reference)? - 4.36. Q. Please explain the rules for local and global variables in Python. - 4.37. Q. How can I have modules that mutually import each other? - 4.38. Q. How do I copy an object in Python? - 4.39. Q. How to implement persistent objects in Python? (Persistent == - automatically saved to and restored from disk.) - 4.40. Q. I try to use __spam and I get an error about _SomeClassName__spam. - 4.41. Q. How do I delete a file? And other file questions. - 4.42. Q. How to modify urllib or httplib to support HTTP/1.1? - 4.43. Q. Unexplicable syntax errors in compile() or exec. - 4.44. Q. How do I convert a string to a number? - 4.45. Q. How do I convert a number to a string? - - 5. Extending Python - 5.1. Q. Can I create my own functions in C? - 5.2. Q. Can I create my own functions in C++? - 5.3. Q. How can I execute arbitrary Python statements from C? - 5.4. Q. How can I evaluate an arbitrary Python expression from C? - 5.5. Q. How do I extract C values from a Python object? - 5.6. Q. How do I use mkvalue() to create a tuple of arbitrary length? - 5.7. Q. How do I call an object's method from C? - 5.8. Q. How do I catch the output from print_error()? - 5.9. Q. How do I access a module written in Python from C? - 5.10. Q. How do I interface to C++ objects from Python? - - 6. Python's design - 6.1. Q. Why isn't there a switch or case statement in Python? - 6.2. Q. Why does Python use indentation for grouping of statements? - 6.3. Q. Why are Python strings immutable? - 6.4. Q. Why don't strings have methods like index() or sort(), like - lists? - 6.5. Q. Why does Python use methods for some functionality - (e.g. list.index()) but functions for other (e.g. len(list))? - 6.6. Q. Why can't I derive a class from built-in types (e.g. lists or - files)? - 6.7. Q. Why must 'self' be declared and used explicitly in method - definitions and calls? - 6.8. Q. Can't you emulate threads in the interpreter instead of - relying on an OS-specific thread implementation? - 6.9. Q. Why can't lambda forms contain statements? - 6.10. Q. Why don't lambdas have access to variables defined in the - containing scope? - 6.11. Q. Why can't recursive functions be defined inside other functions? - 6.12. Q. Why is there no more efficient way of iterating over a dictionary - than first constructing the list of keys()? - 6.13. Q. Can Python be compiled to machine code, C or some other language? - 6.14. Q. Why doesn't Python use proper garbage collection? - - 7. Using Python on non-UNIX platforms - 7.1. Q. Is there a Mac version of Python? - 7.2. Q. Are there DOS and Windows versions of Python? - 7.3. Q. Is there an OS/2 version of Python? - 7.4. Q. Is there a VMS version of Python? - 7.5. Q. What about IBM mainframes, or other non-UNIX platforms? - 7.6. Q. Where are the source or Makefiles for the non-UNIX versions? - 7.7. Q. What is the status and support for the non-UNIX versions? - 7.8. Q. I have a PC version but it appears to be only a binary. - Where's the library? - 7.9. Q. Where's the documentation for the Mac or PC version? - 7.10. Q. The Mac (PC) version doesn't seem to have any facilities for - creating or editing programs apart from entering it interactively, and - there seems to be no way to save code that was entered interactively. - How do I create a Python program on the Mac (PC)? - -To find a particular question, search for the question number followed -by a dot, a space, and a Q at the beginning of a line (e.g. to find -question 4.2 in vi, type /^4\.2\. Q/). - - -1. General information and availability - ======================================= - -1.1. Q. What is Python? - -A. Python is an interpreted, interactive, object-oriented programming -language. It incorporates modules, exceptions, dynamic typing, very -high level dynamic data types, and classes. Python combines -remarkable power with very clear syntax. It has interfaces to many -system calls and libraries, as well as to various window systems, and -is extensible in C or C++. It is also usable as an extension language -for applications that need a programmable interface. Finally, Python -is portable: it runs on many brands of UNIX, on the Mac, and on PCs -under MS-DOS, Windows, Windows NT, and OS/2. - -To find out more, the best thing to do is to start reading the -tutorial from the documentation set (see a few questions further -down). - -1.2. Q. Why is it called Python? - -A. Apart from being a computer scientist, I'm also a fan of "Monty -Python's Flying Circus" (a BBC comedy series from the seventies, in -the -- unlikely -- case you didn't know). It occurred to me one day -that I needed a name that was short, unique, and slightly mysterious. -And I happened to be reading some scripts from the series at the -time... So then I decided to call my language Python. But Python is -not a joke. And don't you associate it with dangerous reptiles -either! (If you need an icon, use an image of the 16-ton weight from -the TV series or of a can of SPAM :-) - -1.3. Q. How do I obtain a copy of the Python source? - -A. The latest complete Python source distribution is always available -by anonymous ftp, e.g. -<URL:ftp://ftp.python.org/pub/python/src/python1.3.tar.gz>. It is a -gzipped tar file containing the complete C source, LaTeX -documentation, Python library modules, example programs, and several -useful pieces of freely distributable software. This will compile and -run out of the box on most UNIX platforms. (See section 7 for -non-UNIX information.) - -Sometimes beta versions of a newer release are available; check the -subdirectory "beta" of the above-mentioned URL (i.e. -<URL:ftp://ftp.python.org/pub/python/src/beta/>). (At the time of -writing, beta3 for Python 1.4 is available there, and should be -checked before reporting problems with version 1.3.) - -Occasionally a set of patches is issued which has to be applied using -the patch program. These patches are placed in the same directory, -e.g. <URL:ftp://ftp.python.org/pub/python/src/patch1.1.1>. (At the time -of writing, no patches exist.) - -An index of said ftp directory can be found in the file INDEX. An -HTML version of the index can be found in the file index.html, -<URL:ftp://ftp.python.org/pub/python/index.html>. - -1.4. Q. How do I get documentation on Python? - -A. The LaTeX source for the documentation is part of the source -distribution. If you don't have LaTeX, the latest Python -documentation set is always available by anonymous ftp, e.g. -<URL:ftp://ftp.python.org/pub/python/doc/postscript.tar.gz>. It is a -gzipped tar file containing PostScript files of the reference manual, -the library manual, and the tutorial. Note that the library manual is -the most important one of the set, as much of Python's power stems -from the standard or built-in types, functions and modules, all of -which are described here. PostScript for a high-level description of -Python is in the file nluug-paper.ps (a separate file on the ftp -site). - -1.5. Q. Are there other ftp sites that mirror the Python distribution? - -A. The following anonymous ftp sites keep mirrors of the Python -distribution: - -USA: - - <URL:ftp://ftp.python.org/pub/python/> - <URL:ftp://gatekeeper.dec.com/pub/plan/python/> - <URL:ftp://ftp.uu.net/languages/python/> - <URL:ftp://ftp.wustl.edu/graphics/graphics/sgi-stuff/python/> - <URL:ftp://ftp.sterling.com/programming/languages/python/> - <URL:ftp://uiarchive.cso.uiuc.edu/pub/lang/python/> - <URL:ftp://ftp.pht.com/mirrors/python/python/> - <URL:ftp://ftp.cdrom.com/pub/python/> - -Europe: - - <URL:ftp://ftp.cwi.nl/pub/python/> - <URL:ftp://ftp.funet.fi/pub/languages/python/> - <URL:ftp://ftp.sunet.se/pub/lang/python/> - <URL:ftp://unix.hensa.ac.uk/mirrors/uunet/languages/python/> - <URL:ftp://ftp.ibp.fr/pub/python/> - <URL:ftp://sunsite.cnlab-switch.ch/mirror/python/> - <URL:ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/python/> - -Australia: - - <URL:ftp://ftp.dstc.edu.au/pub/python/> - -Or try archie on the string "python". - -1.6. Q. Is there a newsgroup or mailing list devoted to Python? - -A. There is a newsgroup, comp.lang.python <URL:news:comp.lang.python>, -and a mailing list. The newsgroup and mailing list are gatewayed into -each other -- if you can read news it's unnecessary to subscribe to -the mailing list. Send e-mail to <python-list-request@cwi.nl> to -(un)subscribe to the mailing list. Hypermail archives of (nearly) -everything posted to the mailing list (and thus the newsgroup) are -available on our WWW server, -<URL:http://www.cwi.nl/~guido/hypermail/index.html>. The raw archives -are also available by ftp, e.g. -<URL:ftp://ftp.python.org/pub/python/mail/mailinglist.gz>. The -uncompressed versions of these files can be read with the standard -UNIX Mail program ("Mail -f file") or with nn ("nn file"). To read -them using MH, you could use "inc -file file". (The archival service -has stopped archiving new articles around the end of April 1995. I -hope to revive it on the PSA server www.python.org sometime in the -future.) - -1.7. Q. Is there a WWW page devoted to Python? - -A. Yes, <URL:http://www.python.org/> is the official Python home page. -At the time of writing, this page is not yet completely operational; -you may have a look at the old Python home page: -<URL:http://www.cwi.nl/~guido/Python.html> or at the U.S. copy: -<URL:http://www.python.org/~guido/Python.html>. - -1.8. Q. Is the Python documentation available on the WWW? - -A. Yes, see <URL:http://www.python.org/> (Python's home page). It -contains pointers to hypertext versions of the whole documentation set -(as hypertext, not just PostScript). - -If you wish to browse this collection of HTML files on your own -machine, it is available bundled up by anonymous ftp, -e.g. <URL:ftp://ftp.python.org/pub/python/doc/html.tar.gz>. - -An Emacs-INFO set containing the library manual is also available by -ftp, e.g. <URL:ftp://ftp.python.org/pub/python/doc/lib-info.tar.gz>. - -1.9. Q. Is there a book on Python, or will there be one out soon? - -A. Mark Lutz is writing a Python book for O'Reilly and Associates, to -be published early 1996. See the outline (in PostScript): -<URL:http://www.python.org/workshops/1995-05/outlinep.eps>. - -1.10. Q. Are there any published articles about Python that I can quote? - -A. So far the only refereed and published article that describes -Python in some detail is: - - Guido van Rossum and Jelke de Boer, "Interactively Testing Remote - Servers Using the Python Programming Language", CWI Quarterly, Volume - 4, Issue 4 (December 1991), Amsterdam, pp 283-303. - -LaTeX source for this paper is available as part of the Python source -distribution. - -See also the next section (supposedly Aaron Watters' paper has been -refereed). - -1.11. Q. Are there short introductory papers or talks on Python? - -A. A recent, very entertaining introduction to Python is the tutorial by -Aaron Watters in UnixWorld Online: - - Aaron R. Watters: "The What, Why, Who, and Where of Python", - <URL:http://www.wcmh.com/uworld/archives/95/tutorial/005.html> - -An olded paper is: - - Guido van Rossum, "An Introduction to Python for UNIX/C - Programmers", in the proceedings of the NLUUG najaarsconferentie - 1993 (dutch UNIX users group meeting November 1993). - -PostScript for this paper and for the slides used for the accompanying -presentation is available by ftp as -<URL:ftp://ftp.python.org/pub/python/doc/nluug-paper.ps> and -<URL:ftp://ftp.python.org/pub/python/doc/nluug-slides.ps>, respectively. - -Slides for a talk on Python that I gave at the Usenix Symposium on -Very High Level Languages in Santa Fe, NM, USA in October 1994 are -available as <URL:ftp://ftp.python.org/pub/python/doc/vhll-slides.ps>. - -1.12. Q. How does the Python version numbering scheme work? - -A. Python versions are numbered A.B.C or A.B. A is the major version -number -- it is only incremented for major changes in functionality or -source structure. B is the minor version number, incremented for less -earth-shattering changes to a release. C is the patchlevel -- it is -incremented for each new patch release. Not all releases have patch -releases. Note that in the past, patches have added significant -changes; in fact the changeover from 0.9.9 to 1.0.0 was the first time -that either A or B changed! - -Beta versions have an additional suffix of "betaN" for some small -number N. Note that (for instance) all versions labeled 1.4betaN -*precede* the actual release of 1.4. 1.4b3 is short for 1.4beta3. - -1.13. Q. How do I get a beta test version of Python? - -A. If there are any beta releases, they are published in the normal -source directory (e.g. <URL:ftp://ftp.python.org/pub/python/src/>). - -1.14. Q. Are there copyright restrictions on the use of Python? - -A. Hardly. You can do anything you want with the source, as long as -you leave the copyrights in, and display those copyrights in any -documentation about Python that you produce. Also, don't use the -author's institute's name in publicity without prior written -permission, and don't hold them responsible for anything (read the -actual copyright for a precise legal wording). - -In particular, if you honor the copyright rules, it's OK to use Python -for commercial use, to sell copies of Python in source or binary form, -or to sell products that enhance Python or incorporate Python (or part -of it) in some form. I would still like to know about all commercial -use of Python! - -1.15. Q. Why was Python created in the first place? - -A. Here's a *very* brief summary of what got me started: - -- I had extensive experience with implementing an interpreted language -in the ABC group at CWI, and from working with this group I had -learned a lot about language design. This is the origin of many -Python features, including the use of indentation for statement -grouping and the inclusion of very-high-level data types (although the -details are all different in Python). - -- I had a number of gripes about the ABC language, but also liked many -of its features. It was impossible to extend the ABC language (or its -implementation) to remedy my complaints -- in fact its lack of -extensibility was one of its biggest problems. - -- I had some experience with using Modula-2+ and talked with the -designers of Modula-3 (and read the M3 report). M3 is the origin of -the syntax and semantics used for exceptions, and some other Python -features. - -- I was working in the Amoeba distributed operating system group at -CWI. We needed a better way to do system administration than by -writing either C programs or Bourne shell scripts, since Amoeba had -its own system call interface which wasn't easily accessible from the -Bourne shell. My experience with error handling in Amoeba made me -acutely aware of the importance of exceptions as a programming -language feature. - -- It occurred to me that a scripting language with a syntax like ABC -but with access to the Amoeba system calls would fill the need. I -realized that it would be foolish to write an Amoeba-specific -language, so I decided that I needed a language that was generally -extensible. - -- During the 1989 Christmas holidays, I had a lot of time on my hand, -so I decided to give it a try. During the next year, while still -mostly working on it in my own time, Python was used in the Amoeba -project with increasing success, and the feedback from colleagues made -me add many early improvements. - -- In February 1991, after just over a year of development, I decided -to post to USENET. The rest is in the Misc/HISTORY file. - - -2. Python in the real world -=========================== - -2.1. Q. How many people are using Python? - -A. I don't know, but the maximum number of simultaneous subscriptions -to the Python mailing list before it was gatewayed into the newsgroup -was about 180 (several of which were local redistribution lists). I -believe that many active Python users don't bother to subscribe to the -list, and now that there's a newsgroup the mailing list subscription -is even less meaningful. I see new names on the newsgroup all the -time and my best guess is that there are currently at least several -thousands of users. - -Another statistic is the number of accesses to the Python WWW server. -Have a look at <URL:http://www.python.org/stats/>. - -2.2. Q. Have any significant projects been done in Python? - -A. Here at CWI (the home of Python), we have written a 20,000 line -authoring environment for transportable hypermedia presentations, a -5,000 line multimedia teleconferencing tool, as well as many many -smaller programs. - -The University of Virginia uses Python to control a virtual reality -engine. Contact: Matt Conway <conway@virginia.edu>. - -The ILU project at Xerox PARC can generate Python glue for ILU -interfaces. See <URL:ftp://ftp.parc.xerox.com/pub/ilu/ilu.html>. - -The University of California, Irvine uses a student administration -system called TELE-Vision written entirely in Python. Contact: Ray -Price <rlprice@uci.edu>. - -See also the next question. - -If you have done a significant project in Python that you'd like to be -included in the list above, send me email! - -2.3. Q. Are there any commercial projects going on using Python? - -A. Yes, there's lots of commercial activity using Python. See -<URL:http://www.python.org/python/Users.html> for a list. - -2.4. Q. How stable is Python? - -A. Very stable. While the current version number would suggest it is -in the early stages of development, in fact new, stable releases -(numbered 0.9.x through 1.4) have been coming out roughly every 3 to -6 or 12 months for the past four years. - -2.5. Q. What new developments are expected for Python in the future? - -A. Follow the newsgroup discussions! The workshop proceedings -(<URL:http://www.python.org/workshops/>) may also contain interesting -looks into the future. - -2.6. Q. Is it reasonable to propose incompatible changes to Python? - -A. In general, no. There are already millions of lines of Python code -around the world, so any changes in the language that invalidates more -than a very small fraction of existing programs has to be frowned -upon. Even if you can provide a conversion program, there still is -the problem of updating all documentation. Providing a gradual -upgrade path is the only way if a feature has to be changed. - -2.7. Q. What is the future of Python? - -A. If I knew, I'd be rich :-) - -Seriously, the formation of the PSA (Pyton Software Activity, see -<URL:http://www.python.org/psa/>) ensures some kind of support even in -the (unlikely! event that I'd be hit by a bus (actually, here in the -US, a car accident would be more likely :-), were to join a nunnery, -or would be head-hunted. A large number of Python users have become -experts at Python programming as well as maintenance of the -implementation, and would easily fill the vacuum created by my -disappearance. - -In the mean time, I have no plans to disappear -- rather, I am -committed to improving Python, and my current benefactor, CNRI (see -<URL:http://www.cnri.reston.va.us>) is just as committed to continue -its support of Python and the PSA. In fact, we have great plans for -Python -- we just can't tell yet! - -2.8. Q. What is the PSA, anyway? - -A. The Python Software Activity <URL:http://www.python.org/psa/> was -created by a number of Python aficionados who want Python to be more -than the product and responsibility of a single individual. It has -found a home at CNRI <URL:http://www.cnri.reston.va.us>. Anybody who -wishes Python well should join the PSA. - -2.9. Q. How do I join the PSA? - -A. The full scoop is available on the web, see -<URL:http://www.python.org/psa/Joining.html>. Summary: send a check -of at least $50 to CNRI/PSA, 1895 Preston White Drive, Suite 100, in -Reston, VA 20191. Full-time students pay $25. Companies can join for -a mere $500. - -2.10. Q. What are the benefits of joining the PSA? - -A. Like National Public Radio, if not enough people join, Python will -wither. Your name will be mentioned on the PSA's web server. -Workshops organized by the PSA <URL:http://www.python.org/workshops/> -are only accessible to PSA members (you can join at the door). The -PSA is working on additional benefits, such as reduced prices for -books and software, and early access to beta versions of Python. - - -3. Building Python and Other Known Bugs -======================================= - -3.1. Q. Is there a test set? - -A. Yes, simply do "import testall" (or "import autotest" if you aren't -interested in the output). The standard modules whose name begins -with "test" together comprise the test. The test set doesn't test -*all* features of Python but it goes a long way to confirm that a new -port is actually working. The Makefile contains an entry "make test" -which runs the autotest module. NOTE: if "make test" fails, run the -tests manually ("import testall") to see what goes wrong before -reporting the error. - -3.2. Q. When running the test set, I get complaints about floating point -operations, but when playing with floating point operations I cannot -find anything wrong with them. - -A. The test set makes occasional unwarranted assumptions about the -semantics of C floating point operations. Until someone donates a -better floating point test set, you will have to comment out the -offending floating point tests and execute similar tests manually. - -3.3. Q. Link errors after rerunning the configure script. - -A. It is generally necessary to run "make clean" after a configuration -change. - -3.4. Q. The python interpreter complains about options passed to a -script (after the script name). - -A. You are probably linking with GNU getopt, e.g. through -liberty. -Don't. The reason for the complaint is that GNU getopt, unlike System -V getopt and other getopt implementations, doesn't consider a -non-option to be the end of the option list. A quick (and compatible) -fix for scripts is to add "--" to the interpreter, like this: - - #! /usr/local/bin/python -- - -You can also use this interactively: - - python -- script.py [options] - -Note that a working getopt implementation is provided in the Python -distribution (in Python/getopt.c) but not automatically used. - -3.5. Q. When building on the SGI, make tries to run python to create -glmodule.c, but python hasn't been built or installed yet. - -A. Comment out the line mentioning glmodule.c in Setup and build a -python without gl first; install it or make sure it is in your $PATH, -then edit the Setup file again to turn on the gl module, and make -again. You don't need to do "make clean"; you do need to run "make -Makefile" in the Modules subdirectory (or just run "make" at the -toplevel). - -3.6. Q. I use VPATH but some targets are built in the source directory. - -A. On some systems (e.g. Sun), if the target already exists in the -source directory, it is created there instead of in the build -directory. This is usually because you have previously built without -VPATH. Try running "make clobber" in the source directory. - -3.7. Q. Trouble building or linking with the GNU readline library. - -A. Consider using readline 2.0. Some hints: - -- You can use the GNU readline library to improve the interactive user -interface: this gives you line editing and command history when -calling python interactively. You need to configure and build the GNU -readline library before running the configure script. Its sources are -no longer distributed with Python; you can ftp them from any GNU -mirror site, or from its home site -<URL:ftp://slc2.ins.cwru.edu/pub/dist/readline-2.0.tar.gz> (or a -higher version number -- using version 1.x is not recommended). Pass -the Python configure script the option --with-readline=DIRECTORY where -DIRECTORY is the absolute pathname of the directory where you've built -the readline library. Some hints on building and using the readline -library: - -- On SGI IRIX 5, you may have to add the following -to rldefs.h: - - #ifndef sigmask - #define sigmask(sig) (1L << ((sig)-1)) - #endif - -- On most systems, you will have to add #include "rldefs.h" to the -top of several source files, and if you use the VPATH feature, you -will have to add dependencies of the form foo.o: foo.c to the -Makefile for several values of foo. - -- The readline library requires use of the termcap library. A -known problem with this is that it contains entry points which -cause conflicts with the STDWIN and SGI GL libraries. The STDWIN -conflict can be solved (and will be, in the next release of -STDWIN) by adding a line saying '#define werase w_erase' to the -stdwin.h file (in the STDWIN distribution, subdirectory H). The -GL conflict has been solved in the Python configure script by a -hack that forces use of the static version of the termcap library. - -- Check the newsgroup gnu.bash.bug <URL:news:gnu.bash.bug> for -specific problems with the readline library (I don't read this group -but I've been told that it is the place for readline bugs). - -3.8. Q. Trouble with socket I/O on older Linux 1.x versions. - -A. Once you've built Python, use it to run the regen.py script in the -Lib/linux1 directory. Apparently the files as distributed don't match -the system headers on some Linux versions. - -3.9. Q. Trouble with prototypes on Ultrix. - -A. Ultrix cc seems broken -- use gcc, or edit config.h to #undef -HAVE_PROTOTYPES. - -3.10. Q. Other trouble building Python on platform X. - -A. Please email the details to <guido@cnri.reston.va.us> and I'll look -into it. Please provide as many details as possible. In particular, -if you don't tell me what type of computer and what operating system -(and version) you are using it will be difficult for me to figure out -what is the matter. If you get a specific error message, please email -it to me too. - -3.11. Q. How to configure dynamic loading on Linux. - -A. This is now automatic as long as your Linux version uses the ELF -object format (all recent Linuxes do). - -3.12. Q: I can't get shared modules to work on Linux 2.0 (Slackware96)? - -A: This is a bug in the Slackware96 release. The fix is simple: - -Make sure that there is a link from /lib/libdl.so to /lib/libdl.so.1 -so that the following links are setup: - - /lib/libdl.so -> /lib/libdl.so.1 - /lib/libdl.so.1 -> /lib/libdl.so.1.7.14 - -You may have to rerun the configure script, after rm'ing the -config.cache file, before you attempt to rebuild python after this -fix. - -3.13. Q: Trouble when making modules shared on Linux. - -A. This happens when you have built Python for static linking and then -enable *shared* in the Setup file. Shared library code must be -compiled with "-fpic". If a .o file for the module already exist that -was compiled for static linking, you must remove it or do "make clean" -in the Modules directory. - -3.14. Q. How to use threads on Linux. - -A. [Greg Stein] I built myself a libpthreads.so from the libc.5.3.12 -distribution (the binary distribution doesn't have pthreads in -it). Then, I configured Python with --with-threads and then tweaked -config.h to include a #define _MIT_POSIX_THREADS (or something like -that, see /usr/include/pthreads.h). It worked fine at that point. - -Note that I couldn't get threading to "operate well" with any of the -other thread packages. Prior libc versions didn't integrate well with -threads, either, so I couldn't use them (e.g. sleep() blocked all -threads :-( ). - -3.15. Q. Errors when linking with a shared library containing C++ code. - -A. Link the main Python binary with C++. Change the definition of -LINKCC in Modules/Makefile to be your C++ compiler. You may have to -edit config.c slightly to make it compilable with C++. - -3.16. Q. I built with tkintermodule.c enabled but get "Tkinter not found". - -A. Tkinter.py (note: upper case T) lives in a subdirectory of Lib, -Lib/tkinter. If you are using the default module search path, you -probably didn't enable the line in the Modules/Setup file defining -TKPATH; if you use the environment variable PYTHONPATH, you'll have to -add the proper tkinter subdirectory. - -3.17. Q. I built with Tk 4.0 but Tkinter complains about the Tk version. - -A. Several things could cause this. You most likely have a Tk 3.6 -installation that wasn't completely eradicated by the Tk 4.0 -installation (which tends to add "4.0" to its installed files). You -may have the Tk 3.6 support library installed in the place where the -Tk 4.0 support files should be (default /usr/local/lib/tk/); you may -have compiled Python with the old tk.h header file (yes, this actually -compiles!); you may actually have linked with Tk 3.6 even though Tk -4.0 is also around. Similar for Tcl 7.4 vs. Tcl 7.3. - -3.18. Q. Link errors for Tcl/Tk symbols when linking with Tcl/Tk. - -Quite possibly, there's a version mismatch between the Tcl/Tk header -files (tcl.h and tk.h) and the tck/tk libraries you are using (the -"-ltk4.0" and "-ltcl7.4" arguments for _tkinter in the Setup file). -If you have installed both versions 7.4/4.0 and 7.5/4.1 of Tcl/Tk, -most likely your header files are for The newer versions, but the -Setup line for _tkinter in some Python distributions references -7.4/4.0 by default. Changing this to 7.5/4.1 should take care of -this. - -3.19. Q. I configured and built Python for Tcl/Tk but "import Tkinter" -fails. - -A. Most likely, you forgot to enable the line in Setup that says -"TKPATH=:$(DESTLIB)/tkinter". - -3.20. Q. Tk doesn't work right on DEC Alpha. - -A. You probably compiled either Tcl, Tk or Python with gcc. Don't. -For this platform, which has 64-bit integers, gcc is known to generate -broken code. The standard cc (which comes bundled with the OS!) -works. If you still prefer gcc, at least try recompiling with cc -before reporting problems to the newsgroup or the author; if this -fixes the problem, report the bug to the gcc developers instead. (As -far as we know, there are no problem with gcc on other platforms -- -the instabilities seem to be restricted to the DEC Alpha.) See also -question 3.6. - -3.21. Q. Several common system calls are missing from the posix module. - -A. Most likely, *all* test compilations run by the configure script -are failing for some reason or another. Have a look in config.log to -see what could be the reason. A common reason is specifying a -directory to the --with-readline option that doesn't contain the -libreadline.a file. - -3.22. Q. ImportError: No module named string, on MS Windows. - -A. Most likely, your PYTHONPATH environment variable should be set to -something like: - -set PYTHONPATH=c:\python;c:\python\lib;c:\python\scripts - -(assuming Python was installed in c:\python) - -3.23. Q. Core dump on SGI when using the gl module. - -There are conflicts between entry points in the termcap and curses -libraries and an entry point in the GL library. There's a hack of a -fix for the termcap library if it's needed for the GNU readline -library, but it doesn't work when you're using curses. Concluding, -you can't build a Python binary containing both the curses and gl -modules. - - - -4. Programming in Python -======================== - -4.1. Q. Is there a source code level debugger with breakpoints, step, -etc.? - -A. Yes. Check out module pdb; pdb.help() prints the documentation (or -you can read it as Lib/pdb.doc). If you use the STDWIN option, -there's also a windowing interface, wdb. You can write your own -debugger by using the code for pdb or wdb as an example. - -4.2. Q. Can I create an object class with some methods implemented in -C and others in Python (e.g. through inheritance)? (Also phrased as: -Can I use a built-in type as base class?) - -A. No, but you can easily create a Python class which serves as a -wrapper around a built-in object, e.g. (for dictionaries): - - # A user-defined class behaving almost identical - # to a built-in dictionary. - class UserDict: - def __init__(self): self.data = {} - def __repr__(self): return repr(self.data) - def __cmp__(self, dict): - if type(dict) == type(self.data): - return cmp(self.data, dict) - else: - return cmp(self.data, dict.data) - def __len__(self): return len(self.data) - def __getitem__(self, key): return self.data[key] - def __setitem__(self, key, item): self.data[key] = item - def __delitem__(self, key): del self.data[key] - def keys(self): return self.data.keys() - def items(self): return self.data.items() - def values(self): return self.data.values() - def has_key(self, key): return self.data.has_key(key) - -A2. See Jim Fulton's ExtensionClass for an example of a mechanism -which allows you to have superclasses which you can inherit from in -Python -- that way you can have some methods from a C superclass (call -it a mixin) and some methods from either a Python superclass or your -subclass. See <URL:http://www.digicool.com/papers/ExtensionClass.html>. - -4.3. Q. Is there a curses/termcap package for Python? - -A. Yes -- Lance Ellinghaus has written a module that interfaces to -System V's "ncurses". If you know a little curses and some Python, -it's straightforward to use. It is part of the standard Python -distribution, but not configured by default -- you must enable it by -editing Modules/Setup. It requires a System V curses implementation. - -You could also consider using the "alfa" (== character cell) version -of STDWIN. (Standard Window System Interface, a portable windowing -system interface by myself <URL:ftp://ftp.cwi.nl/pub/stdwin/>.) This -will also prepare your program for porting to windowing environments -such as X11 or the Macintosh. - -4.4. Q. Is there an equivalent to C's onexit() in Python? - -A. Yes, if you import sys and assign a function to sys.exitfunc, it -will be called when your program exits, is killed by an unhandled -exception, or (on UNIX) receives a SIGHUP or SIGTERM signal. - -4.5. Q. When I define a function nested inside another function, the -nested function seemingly can't access the local variables of the -outer function. What is going on? How do I pass local data to a -nested function? - -A. Python does not have arbitrarily nested scopes. When you need to -create a function that needs to access some data which you have -available locally, create a new class to hold the data and return a -method of an instance of that class, e.g.: - - class MultiplierClass: - def __init__(self, factor): - self.factor = factor - def multiplier(self, argument): - return argument * self.factor - - def generate_multiplier(factor): - return MultiplierClass(factor).multiplier - - twice = generate_multiplier(2) - print twice(10) - # Output: 20 - -An alternative solution uses default arguments, e.g.: - - def generate_multiplier(factor): - def multiplier(arg, fact = factor): - return arg*fact - return multiplier - - twice = generate_multiplier(2) - print twice(10) - # Output: 20 - -4.6. Q. How do I iterate over a sequence in reverse order? - -A. If it is a list, the fastest solution is - - list.reverse() - try: - for x in list: - "do something with x" - finally: - list.reverse() - -This has the disadvantage that while you are in the loop, the list -is temporarily reversed. If you don't like this, you can make a copy. -This appears expensive but is actually faster than other solutions: - - rev = list[:] - rev.reverse() - for x in rev: - <do something with x> - -If it isn't a list, a more general but slower solution is: - - i = len(list) - while i > 0: - i = i-1 - x = list[i] - <do something with x> - -A more elegant solution, is to define a class which acts as a sequence -and yields the elements in reverse order (solution due to Steve -Majewski): - - class Rev: - def __init__(self, seq): - self.forw = seq - def __len__(self): - return len(self.forw) - def __getitem__(self, i): - return self.forw[-(i + 1)] - -You can now simply write: - - for x in Rev(list): - <do something with x> - -Unfortunately, this solution is slowest of all, due to the method -call overhead... - -4.7. Q. My program is too slow. How do I speed it up? - -A. That's a tough one, in general. There are many tricks to speed up -Python code; I would consider rewriting parts in C only as a last -resort. One thing to notice is that function and (especially) method -calls are rather expensive; if you have designed a purely OO interface -with lots of tiny functions that don't do much more than get or set an -instance variable or call another method, you may consider using a -more direct way, e.g. directly accessing instance variables. Also see -the standard module "profile" (described in the file -"python/lib/profile.doc") which makes it possible to find out where -your program is spending most of its time (if you have some patience --- the profiling itself can slow your program down by an order of -magnitude). - -4.8. Q. When I have imported a module, then edit it, and import it -again (into the same Python process), the changes don't seem to take -place. What is going on? - -A. For reasons of efficiency as well as consistency, Python only reads -the module file on the first time a module is imported. (Otherwise a -program consisting of many modules, each of which imports the same -basic module, would read the basic module over and over again.) To -force rereading of a changed module, do this: - - import modname - reload(modname) - -Warning: this technique is not 100% fool-proof. In particular, -modules containing statements like - - from modname import some_objects - -will continue to work with the old version of the imported objects. - -4.9. Q. How do I find the current module name? - -A. A module can find out its own module name by looking at the -(predefined) global variable __name__. If this has the value -'__main__' you are running as a script. - -4.10. Q. I have a module in which I want to execute some extra code -when it is run as a script. How do I find out whether I am running as -a script? - -A. See the previous question. E.g. if you put the following on the -last line of your module, main() is called only when your module is -running as a script: - - if __name__ == '__main__': main() - -4.11. Q. I try to run a program from the Demo directory but it fails -with ImportError: No module named ...; what gives? - -A. This is probably an optional module (written in C!) which hasn't -been configured on your system. This especially happens with modules -like "Tkinter", "stdwin", "gl", "Xt" or "Xm". For Tkinter, STDWIN and -many other modules, see Modules/Setup.in for info on how to add these -modules to your Python, if it is possible at all. Sometimes you will -have to ftp and build another package first (e.g. STDWIN). Sometimes -the module only works on specific platforms (e.g. gl only works on SGI -machines). - -NOTE: if the complaint is about "Tkinter" (upper case T) and you have -already configured module "tkinter" (lower case t), the solution is -*not* to rename tkinter to Tkinter or vice versa. There is probably -something wrong with your module search path. Check out the value of -sys.path. - -For X-related modules (Xt and Xm) you will have to do more work: they -are currently not part of the standard Python distribution. You will -have to ftp the Extensions tar file, e.g. -<URL:ftp://ftp.python.org/pub/python/src/extensions.tar.gz> and follow -the instructions there. - -See also the next question. - -4.12. Q. I have successfully built Python with STDWIN but it can't -find some modules (e.g. stdwinevents). - -A. There's a subdirectory of the library directory named 'stdwin' -which should be in the default module search path. There's a line in -Modules/Setup(.in) that you have to enable for this purpose -- -unfortunately in the latest release it's not near the other -STDWIN-related lines so it's easy to miss it. - -4.13. Q. What GUI toolkits exist for Python? - -A. Depending on what platform(s) you are aiming at, there are several. - -Currently supported solutions: - -- There's a neat object-oriented interface to the Tcl/Tk widget set, -called Tkinter. It is part of the standard Python distribution and -well-supported -- all you need to do is build and install Tcl/Tk and -enable the _tkinter module and the TKPATH definition in Modules/Setup -when building Python. This is probably the easiest to install and -use, and the most complete widget set. It is also very likely that in -the future the standard Python GUI API will be based on or at least -look very much like the Tkinter interface. For more info about Tk, -including pointers to the source, see the Tcl/Tk home page -<URL:http://www.sunlabs.com/research/tcl/>. Tcl/Tk is now fully -portable to the Mac and Windows platforms (NT and 95 only); you need -Python 1.4beta3 or later and Tk 4.1patch1 or later. - -- There's an interface to X11, including the Athena and Motif widget -sets (and a few individual widgets, like Mosaic's HTML widget and -SGI's GL widget) available from -<URL:ftp://ftp.python.org/pub/python/src/X-extension.tar.gz>. -Support by Sjoerd Mullender <sjoerd@cwi.nl>. - -- On top of the X11 interface there's the (recently revived) vpApp -toolkit by Per Spilling, now also maintained by Sjoerd Mullender -<sjoerd@cwi.nl>. See <URL:ftp://ftp.cwi.nl/pub/sjoerd/vpApp.tar.gz>. - -- The Mac port has a rich and ever-growing set of modules that support -the native Mac toolbox calls. See the documentation that comes with -the Mac port. See <URL:ftp://ftp.python.org/pub/python/mac>. Support -by Jack Jansen <jack@cwi.nl>. - -- The NT port supported by Mark Hammond <MHammond@skippinet.com.au> -(see question 7.2) includes an interface to the Microsoft Foundation -Classes and a Python programming environment using it that's written -mostly in Python. See -<URL:ftp://ftp.python.org/pub/python/pythonwin/>. - -- There's an object-oriented GUI based on the Microsoft Foundation -Classes model called WPY, supported by Jim Ahlstrom <jim@interet.com>. -Programs written in WPY run unchanged and with native look and feel on -Windows NT/95, Windows 3.1 (using win32s), and on Unix (using Tk). -Source and binaries for Windows and Linux are available in -<URL:ftp://ftp.python.org/pub/python/wpy/>. - -Obsolete or minority solutions: - -- There's an interface to wxWindows. wxWindows is a portable GUI -class library written in C++. It supports XView, Motif, MS-Windows as -targets. There is some support for Macs and CURSES as well. -wxWindows preserves the look and feel of the underlying graphics -toolkit. See the wxPython WWW page at -<URL:http://www.aiai.ed.ac.uk/~jacs/wx/wxpython/wxpython.html>. -Support for wxPython (by Harri Pasanen <pa@tekla.fi>) appears -to have a low priority. - -- For SGI IRIX only, there are unsupported interfaces to the complete -GL (Graphics Library -- low level but very good 3D capabilities) as -well as to FORMS (a buttons-and-sliders-etc package built on top of GL -by Mark Overmars -- ftp'able from -<URL:ftp://ftp.cs.ruu.nl/pub/SGI/FORMS/>). This is probably also -becoming obsolete, as OpenGL takes over. - -- There's an interface to STDWIN, a platform-independent low-level -windowing interface for Mac and X11. This is totally unsupported and -rapidly becoming obsolete. The STDWIN sources are at -<URL:ftp://ftp.cwi.nl/pub/stdwin/>. (For info about STDWIN 2.0, -please refer to Steven Pemberton <steven@cwi.nl> -- I believe it is -also dead.) - -- There once was an interface to WAFE, a Tcl interface to the X11 -Motif and Athena widget sets. WAFE is at -<URL:ftp://ftp.wu-wien.ac.at/pub/src/X11/wafe/>. It's not clear what -the status of the Python support is. - -- (The Fresco port that was mentioned in earlier versions of this FAQ -no longer seems to exist. Inquire with Mark Linton.) - -4.14. Q. Are there any interfaces to database packages in Python? - -A. There's a whole collection of them in the contrib area of the ftp -server, see <URL:http://www.python.org/ftp/python/contrib/Database/>. - -4.15. Q. Is it possible to write obfuscated one-liners in Python? - -A. Yes. See the following three examples, due to Ulf Bartelt: - - # Primes < 1000 - print filter(None,map(lambda y:y*reduce(lambda x,y:x*y!=0, - map(lambda x,y=y:y%x,range(2,int(pow(y,0.5)+1))),1),range(2,1000))) - - # First 10 Fibonacci numbers - print map(lambda x,f=lambda x,f:(x<=1) or (f(x-1,f)+f(x-2,f)): f(x,f), - range(10)) - - # Mandelbrot set - print (lambda Ru,Ro,Iu,Io,IM,Sx,Sy:reduce(lambda x,y:x+y,map(lambda y, - Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,Sy=Sy,L=lambda yc,Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,i=IM, - Sx=Sx,Sy=Sy:reduce(lambda x,y:x+y,map(lambda x,xc=Ru,yc=yc,Ru=Ru,Ro=Ro, - i=i,Sx=Sx,F=lambda xc,yc,x,y,k,f=lambda xc,yc,x,y,k,f:(k<=0)or (x*x+y*y - >=4.0) or 1+f(xc,yc,x*x-y*y+xc,2.0*x*y+yc,k-1,f):f(xc,yc,x,y,k,f):chr( - 64+F(Ru+x*(Ro-Ru)/Sx,yc,0,0,i)),range(Sx))):L(Iu+y*(Io-Iu)/Sy),range(Sy - ))))(-2.1, 0.7, -1.2, 1.2, 30, 80, 24) - # \___ ___/ \___ ___/ | | |__ lines on screen - # V V | |______ columns on screen - # | | |__________ maximum of "iterations" - # | |_________________ range on y axis - # |____________________________ range on x axis - -Don't try this at home, kids! - -4.16. Q. Is there an equivalent of C's "?:" ternary operator? - -A. Not directly. In many cases you can mimic a?b:c with "a and b or -c", but there's a flaw: if b is zero (or empty, or None -- anything -that tests false) then c will be selected instead. In many cases you -can prove by looking at the code that this can't happen (e.g. because -b is a constant or has a type that can never be false), but in general -this can be a problem. - -Tim Peters (who wishes it was Steve Majewski) suggested the following -solution: (a and [b] or [c])[0]. Because [b] is a singleton list it -is never false, so the wrong path is never taken; then applying [0] to -the whole thing gets the b or c that you really wanted. Ugly, but it -gets you there in the rare cases where it is really inconvenient to -rewrite your code using 'if'. - -4.17. Q. My class defines __del__ but it is not called when I delete the -object. - -A. There are several possible reasons for this. - -- The del statement does not necessarily call __del__ -- it simply -decrements the object's reference count, and if this reaches zero -__del__ is called. - -- If your data structures contain circular links (e.g. a tree where -each child has a parent pointer and each parent has a list of -children) the reference counts will never go back to zero. You'll -have to define an explicit close() method which removes those -pointers. Please don't ever call __del__ directly -- __del__ should -call close() and close() should make sure that it can be called more -than once for the same object. - -- If the object has ever been a local variable (or argument, which is -really the same thing) to a function that caught an expression in an -except clause, chances are that a reference to the object still exists -in that function's stack frame as contained in the stack trace. -Normally, deleting (better: assigning None to) sys.exc_traceback will -take care of this. If you a stack was printed for an unhandled -exception in an interactive interpreter, delete sys.last_traceback -instead. - -- There is code that deletes all objects when the interpreter exits, -but if your Python has been configured to support threads, it is not -called (because other threads may still be active). You can define -your own cleanup function using sys.exitfunc (see question 4.4). - -- Finally, if your __del__ method raises an exception, this will be -ignored. Starting with Python 1.4beta3, a warning message is printed -to sys.stderr when this happens. - -4.18. Q. How do I change the shell environment for programs called -using os.popen() or os.system()? Changing os.environ doesn't work. - -A. You must be using either a version of python before 1.4, or on a -(rare) system that doesn't have the putenv() library function. - -Before Python 1.4, modifying the environment passed to subshells was -left out of the interpreter because there seemed to be no -well-established portable way to do it (in particular, some systems, -have putenv(), others have setenv(), and some have none at all). As -of Python 1.4, almost all Unix systems *do* have putenv(), and so does -the Win32 API, and thus the os module was modified so that changes to -os.environ are trapped and the corresponding putenv() call is made. - -4.19. Q. What is a class? - -A. A class is the particular object type that is created by executing -a class statement. Class objects are used as templates, to create -class instance objects, which embody both the data structure and -program routines specific to a datatype. - -4.20. Q. What is a method? - -A. A method is a function that you normally call as -x.name(arguments...) for some object x. The term is used for methods -of classes and class instances as well as for methods of built-in -objects. (The latter have a completely different implementation and -only share the way their calls look in Python code.) Methods of -classes (and class instances) are defined as functions inside the -class definition. - -4.21. Q. What is self? - -A. Self is merely a conventional name for the first argument of a -method -- i.e. a function defined inside a class definition. A method -defined as meth(self, a, b, c) should be called as x.meth(a, b, c) for -some instance x of the class in which the definition occurs; -the called method will think it is called as meth(x, a, b, c). - -4.22. Q. What is a unbound method? - -A. An unbound method is a method defined in a class that is not yet -bound to an instance. You get an unbound method if you ask for a -class attribute that happens to be a function. You get a bound method -if you ask for an instance attribute. A bound method knows which -instance it belongs to and calling it supplies the instance automatically; -an unbound method only knows which class it wants for its first -argument (a derived class is also OK). Calling an unbound method -doesn't "magically" derive the first argument from the context -- you -have to provide it explicitly. - -4.23. Q. How do I call a method defined in a base class from a derived -class that overrides it? - -A. If your class definition starts with "class Derived(Base): ..." -then you can call method meth defined in Base (or one of Base's base -classes) as Base.meth(self, arguments...). Here, Base.meth is an -unbound method (see previous question). - -4.24. Q. How do I call a method from a base class without using the -name of the base class? - -A. DON'T DO THIS. REALLY. I MEAN IT. It appears that you could call -self.__class__.__bases__[0].meth(self, arguments...) but this fails when -a doubly-derived method is derived from your class: for its instances, -self.__class__.__bases__[0] is your class, not its base class -- so -(assuming you are doing this from within Derived.meth) you would start -a recursive call. - -4.25. Q. How can I organize my code to make it easier to change the base -class? - -A. You could define an alias for the base class, assign the real base -class to it before your class definition, and use the alias throughout -your class. Then all you have to change is the value assigned to the -alias. Incidentally, this trick is also handy if you want to decide -dynamically (e.g. depending on availability of resources) which base -class to use. Example: - - BaseAlias = <real base class> - class Derived(BaseAlias): - def meth(self): - BaseAlias.meth(self) - ... - -4.26. Q. How can I find the methods or attributes of an object? - -A. This depends on the object type. - -For an instance x of a user-defined class, instance attributes are -found in the dictionary x.__dict__, and methods and attributes defined -by its class are found in x.__class__.__bases__[i].__dict__ (for i in -range(len(x.__class__.__bases__))). You'll have to walk the tree of -base classes to find *all* class methods and attributes. - -Many, but not all built-in types define a list of their method names -in x.__methods__, and if they have data attributes, their names may be -found in x.__members__. However this is only a convention. - -For more information, read the source of the standard (but -undocumented) module newdir. - -4.27. Q. I can't seem to use os.read() on a pipe created with os.popen(). - -A. os.read() is a low-level function which takes a file descriptor (a -small integer). os.popen() creates a high-level file object -- the -same type used for sys.std{in,out,err} and returned by the builtin -open() function. Thus, to read n bytes from a pipe p created with -os.popen(), you need to use p.read(n). - -4.28. Q. How can I create a stand-alone binary from a Python script? - -The demo script "Demo/scripts/freeze.py" does what you want. (It's -actually not a demo but a support tool -- there is some extra code in -the interpreter to accommodate it.) It requires that you have the -Python build tree handy, complete with all the lib*.a files. - -This works by scanning your source recursively for import statements -(both forms) and looking for the modules on the standard Python path -as well as in the source directory (for built-in modules). It then -"compiles" the modules written in Python to C code (array initializers -that can be turned into code objects using the marshal module) and -creates a custom-made config file that only contains those built-in -modules which are actually used in the program. It then compiles the -generated C code and links it with the rest of the Python interpreter -to form a self-contained binary which acts exactly like your script. - -Hint: the freeze program only works if your script's filename ends in -".py". - -4.29. Q. What WWW tools are there for Python? - -A. See the chapter titled "Internet and WWW" in the Library Reference -Manual. There's also a web browser written in Python, called Grail -- -see <URL:http://grail.cnri.reston.va.us/grail/>. - -Steve Miale <smiale@cs.indiana.edu> has written a modular WWW browser -called Dancer. An alpha version can be FTP'ed from -<URL:ftp://ftp.cs.indiana.edu/pub/smiale/dancer.tar.gz>. (There are a -few articles about Dancer in the (hyper)mail archive -<URL:http://www.cwi.nl/~guido/hypermail/python-1994q3/index.html>.) - -4.30. Q. How do I run a subprocess with pipes connected to both input -and output? - -A. This is really a UNIX question. Also, in general, it is unwise to -do so, because you can easily cause a deadlock where the parent -process is blocked waiting for output from the child, while the child -is blocked waiting for input from the child. This can be caused -because the parent expects the child to output more text than it does, -or it can be caused by data being stuck in stdio buffers due to lack -of flushing. The Python parent can of course explicitly flush the data -it sends to the child before it reads any output, but if the child is -a naive C program it can easily have been written to never explicitly -flush its output, even if it is interactive, since flushing is -normally automatic. - -In many cases, all you really need is to run some data through a -command and get the result back. Unless the data is infinite in size, -the easiest (and often the most efficient!) way to do this is to write -it to a temporary file and run the command with that temporary file as -input. The standard module tempfile exports a function mktemp() which -generates unique temporary file names. - -If after reading all of the above you still want to connect two pipes -to a subprocess's standard input and output, here's a simple solution, -due to Jack Jansen: - - import os - import sys - import string - - MAXFD = 100 # Max number of file descriptors in this system - - def popen2(cmd): - cmd = string.split(cmd) - p2cread, p2cwrite = os.pipe() - c2pread, c2pwrite = os.pipe() - pid = os.fork() - if pid == 0: - # Child - os.close(0) - os.close(1) - if os.dup(p2cread) != 0: - sys.stderr.write('popen2: bad read dup\n') - if os.dup(c2pwrite) != 1: - sys.stderr.write('popen2: bad write dup\n') - for i in range(3, MAXFD): - try: - os.close(i) - except: - pass - try: - os.execv(cmd[0], cmd) - finally: - os._exit(1) - os.close(p2cread) - tochild = os.fdopen(p2cwrite, 'w') - os.close(c2pwrite) - fromchild = os.fdopen(c2pread, 'r') - return fromchild, tochild - -Note that many interactive programs (e.g. vi) don't work well with -pipes substituted for standard input and output. You will have to use -pseudo ttys ("ptys") instead of pipes. There is some undocumented -code to use these in the library module pty.py -- I'm afraid you're on -your own here. - -A different answer is a Python interface to Don Libes' "expect" -library. A prerelease of this is available on the Python ftp mirror -sites in the contrib subdirectory as expy-0.3.tar.gz, e.g. -<URL:ftp://ftp.python.org/pub/python/contrib/expy-0.3.tar.gz>. - -4.31. Q. How do I call a function if I have the arguments in a tuple? - -A. Use the built-in function apply(). For instance, - - func(1, 2, 3) - -is equivalent to - - args = (1, 2, 3) - apply(func, args) - -Note that func(args) is not the same -- it calls func() with exactly -one argument, the tuple args, instead of three arguments, the integers -1, 2 and 3. - -4.32. Q. How do I enable font-lock-mode for Python in Emacs? - -A. Assuming you're already using python-mode and font-lock-mode -separately, all you need to do is put this in your .emacs file: - - (defun my-python-mode-hook () - (setq font-lock-keywords python-font-lock-keywords) - (font-lock-mode 1)) - (add-hook 'python-mode-hook 'my-python-mode-hook) - -4.33. Q. Is there an inverse to the format operator (a la C's scanf())? - -A. Not as such. - -For simple input parsing, the easiest approach is usually to split -the line into whitespace-delimited words using string.split(), and to -convert decimal strings to numeric values using string.atoi(), -string.atol() or string.atof(). (Python's atoi() is 32-bit and its -atol() is arbitrary precision.) If you want to use another delimiter -than whitespace, use string.splitfield() (possibly combining it with -string.strip() which removes surrounding whitespace from a string). - -For more complicated input parsing, regular expressions (see module -regex) are better suited and more powerful than C's scanf(). - -4.34. Q. Can I have Tk events handled while waiting for I/O? - -A. Yes, and you don't even need threads! But you'll have to -restructure your I/O code a bit. Tk has the equivalent of Xt's -XtAddInput() call, which allows you to register a callback function -which will be called from the Tk mainloop when I/O is possible on a -file descriptor. Here's what you need: - - from Tkinter import tkinter - tkinter.createfilehandler(file, mask, callback) - -The file may be a Python file or socket object (actually, anything -with a fileno() method), or an integer file descriptor. The mask is -one of the constants tkinter.READABLE or tkinter.WRITABLE. The -callback is called as follows: - - callback(file, mask) - -You must unregister the callback when you're done, using - - tkinter.deletefilehandler(file) - -Note: since you don't know *how many bytes* are available for reading, -you can't use the Python file object's read or readline methods, since -these will insist on reading a predefined number of bytes. For -sockets, the recv() or recvfrom() methods will work fine; for other -files, use os.read(file.fileno(), maxbytecount). - -4.35. Q. How do I write a function with output parameters (call by reference)? - -A. [Mark Lutz] The thing to remember is that arguments are passed by -assignment in Python. Since assignment just creates references to -objects, there's no alias between an argument name in the caller and -callee, and so no call-by-reference per se. But you can simulate it -in a number of ways: - -1) By using global variables; but you probably shouldn't :-) - -2) By passing a mutable (changeable in-place) object: - - def func1(a): - a[0] = 'new-value' # 'a' references a mutable list - a[1] = a[1] + 1 # changes a shared object - - args = ['old-value', 99] - func1(args) - print args[0], args[1] # output: new-value 100 - -3) By return a tuple, holding the final values of arguments: - - def func2(a, b): - a = 'new-value' # a and b are local names - b = b + 1 # assigned to new objects - return a, b # return new values - - x, y = 'old-value', 99 - x, y = func2(x, y) - print x, y # output: new-value 100 - -4) And other ideas that fall-out from Python's object model. For - instance, it might be clearer to pass in a mutable dictionary: - - def func3(args): - args['a'] = 'new-value' # args is a mutable dictionary - args['b'] = args['b'] + 1 # change it in-place - - args = {'a':' old-value', 'b': 99} - func3(args) - print args['a'], args['b'] - -5) Or bundle-up values in a class instance: - - class callByRef: - def __init__(self, **args): - for (key, value) in args.items(): - setattr(self, key, value) - - def func4(args): - args.a = 'new-value' # args is a mutable callByRef - args.b = args.b + 1 # change object in-place - - args = callByRef(a='old-value', b=99) - func4(args) - print args.a, args.b - - But there's probably no good reason to get this complicated :-). - -[Python' author favors solution 3 in most cases.] - -4.36. Q. Please explain the rules for local and global variables in Python. - -A. [Ken Manheimer] In Python, procedure variables are implicitly -global, unless they assigned anywhere within the block. In that case -they are implicitly local, and you need to explicitly declare them as -'global'. - -Though a bit surprising at first, a moments consideration explains -this. On one hand, requirement of 'global' for assigned vars provides -a bar against unintended side-effects. On the other hand, if global -were required for all global references, you'd be using global all the -time. Eg, you'd have to declare as global every reference to a -builtin function, or to a component of an imported module. This -clutter would defeat the usefulness of the 'global' declaration for -identifying side-effects. - -4.37. Q. How can I have modules that mutually import each other? - -A. Jim Roskind recommends the following order in each module: - -First: all exports (like globals, functions, and classes that don't -need imported bases classes). - -Then: all import statements. - -Finally: all active code (including globals that are initialized from -imported values). - -Python's author doesn't like this approach much because the imports -appear in a strange place, but has to admit that it works. His -recommended strategy is to avoid all uses of "from <module> import *" -(so everything from an imported module is referenced as -<module>.<name>) and to place all code inside functions. -Initializations of global variables and class variables should use -constants or built-in functions only. - -4.38. Q. How do I copy an object in Python? - -A. There is no generic copying operation built into Python, however -most object types have some way to create a clone. Here's how for the -most common objects: - -- For immutable objects (numbers, strings, tuples), cloning is -unnecessary since their value can't change. - -- For lists (and generally for mutable sequence types), a clone is -created by the expression l[:]. - -- For dictionaries, the following function returns a clone: - - def dictclone(o): - n = {} - for k in o.keys(): n[k] = o[k] - return n - -- Finally, for generic objects, the "copy" module defines two -functions for copying objects. copy.copy(x) returns a copy as shown -by the above rules. copy.deepcopy(x) also copies the elements of -composite objects. See the section on this module in the Library -Reference Manual. - -4.39. Q. How to implement persistent objects in Python? (Persistent == -automatically saved to and restored from disk.) - -A. The library module "pickle" now solves this in a very general way -(though you still can't store things like open files, sockests or -windows), and the library module "shelve" uses pickle and (g)dbm to -create presistent mappings containing arbitrary Python objects. - -4.40. Q. I try to use __spam and I get an error about _SomeClassName__spam. - -A. Variables with double leading underscore are "mangled" to provide a -simple but effective way to define class private variables. See the -chapter "New in Release 1.4" in the Python Tutorial. - -4.41. Q. How do I delete a file? And other file questions. - -A. Use os.remove(filename) or os.unlink(filename); for documentation, -see the posix section of the library manual. They are the same, -unlink() is simply the Unix name for this function. In earlier -versions of Python, only os.unlink() was available. - -To remove a directory, use os.rmdir(); use os.mkdir() to create one. - -To rename a file, use os.rename(). - -To truncate a file, open it using f = open(filename, "w+"), and use -f.truncate(offset); offset defaults to the current seek position. -There's also os.ftruncate(fd, offset) for files opened with os.open() --- for advanced Unix hacks only. - -4.42. Q. How to modify urllib or httplib to support HTTP/1.1? - -A. Apply the following patch to httplib.py: - -41c41 -< replypat = regsub.gsub('\\.', '\\\\.', HTTP_VERSION) + \ ---- -> replypat = regsub.gsub('\\.', '\\\\.', 'HTTP/1.[0-9]+') + \ - -4.43. Q. Unexplicable syntax errors in compile() or exec. - -A. When a statement suite (as opposed to an expression) is compiled by -compile(), exec or execfile(), it *must* end in a newline. In some -cases, when the source ends in an indented block it appears that at -least two newlines are required. - -4.44. Q. How do I convert a string to a number? - -A. To convert, e.g., the string '144' to the number 144, import the -module string and use the string.atoi() function. For floating point -numbers, use string.atof(); for long integers, use string.atol(). See -the library reference manual section for the string module for more -details. While you could use the built-in function eval() instead of -any of those, this is not recommended, because someone could pass you -a Python expression that might have unwanted side effects (like -reformatting your disk). - -4.45. Q. How do I convert a number to a string? - -A. To convert, e.g., the number 144 to the string '144', use the -built-in function repr() or the backquote notation (these are -equivalent). If you want a hexadecimal or octal representation, use -the built-in functions hex() or oct(), respectively. For fancy -formatting, use the % operator on strings, just like C printf formats, -e.g. "%04d" % 144 yields '0144' and "%.3f" % (1/3.0) yields '0.333'. -See the library reference manual for details. - - -5. Extending Python -=================== - -5.1. Q. Can I create my own functions in C? - -A. Yes, you can create built-in modules containing functions, -variables, exceptions and even new types in C. This is explained in -the document "Extending and Embedding the Python Interpreter" (the -LaTeX file Doc/ext.tex). Also read the chapter on dynamic loading. - -5.2. Q. Can I create my own functions in C++? - -A. Yes, using the C-compatibility features found in C++. Basically -you place extern "C" { ... } around the Python include files and put -extern "C" before each function that is going to be called by the -Python interpreter. Global or static C++ objects with constructors -are probably not a good idea. - -5.3. Q. How can I execute arbitrary Python statements from C? - -A. The highest-level function to do this is run_command() which takes -a single string argument which is executed in the context of module -__main__ and returns 0 for success and -1 when an exception occurred -(including SyntaxError). If you want more control, use run_string(); -see the source for run_command() in Python/pythonrun.c. - -5.4. Q. How can I evaluate an arbitrary Python expression from C? - -A. Call the function run_string() from the previous question with the -start symbol eval_input; it then parses an expression, evaluates it -and returns its value. See exec_eval() in Python/bltinmodule.c. - -5.5. Q. How do I extract C values from a Python object? - -A. That depends on the object's type. If it's a tuple, -gettuplesize(o) returns its length and gettupleitem(o, i) returns its -i'th item; similar for lists with getlistsize(o) and getlistitem(o, -i). For strings, getstringsize(o) returns its length and -getstringvalue(o) a pointer to its value (note that Python strings may -contain null bytes so strlen() is not safe). To test which type an -object is, first make sure it isn't NULL, and then use -is_stringobject(o), is_tupleobject(o), is_listobject(o) etc. - -5.6. Q. How do I use mkvalue() to create a tuple of arbitrary length? - -A. You can't. Use t = newtupleobject(n) instead, and fill it with -objects using settupleitem(t, i, o) -- note that this "eats" a -reference count of o. Similar for lists with newlistobject(n) and -setlistitem(l, i, o). Note that you *must* set all the tuple items to -some value before you pass the tuple to Python code -- -newtupleobject(n) initializes them to NULL, which isn't a valid Python -value. - -5.7. Q. How do I call an object's method from C? - -A. Here's a function (untested) that might become part of the next -release in some form. It uses <stdarg.h> to allow passing the -argument list on to vmkvalue(): - - object *call_method(object *inst, char *methodname, char *format, ...) - { - object *method; - object *args; - object *result; - va_list va; - method = getattr(inst, methodname); - if (method == NULL) return NULL; - va_start(va, format); - args = vmkvalue(format, va); - va_end(va); - if (args == NULL) { - DECREF(method); - return NULL; - } - result = call_object(method, args); - DECREF(method); - DECREF(args); - return result; - } - -This works for any instance that has methods -- whether built-in or -user-defined. You are responsible for eventually DECREF'ing the -return value. - -To call, e.g., a file object's "seek" method with arguments 10, 0 -(assuming the file object pointer is "f"): - - res = call_method(f, "seek", "(OO)", 10, 0); - if (res == NULL) { - ... an exception occurred ... - } - else { - DECREF(res); - } - -Note that since call_object() *always* wants a tuple for the argument -list, to call a function without arguments, pass "()" for the format, -and to call a function with one argument, surround the argument in -parentheses, e.g. "(i)". - -5.8. Q. How do I catch the output from print_error()? - -A. (Due to Mark Hammond): - -* in Python code, define an object that supports the "write()" method. - -* redirect sys.stdout and sys.stderr to this object. - -* call print_error, or just allow the standard traceback mechanism to -work. - -Then, the output will go wherever your write() method sends it. - -5.9. Q. How do I access a module written in Python from C? - -A. You can get a pointer to the module object as follows: - - module = import_module("<modulename>"); - -If the module hasn't been imported yet (i.e. it is not yet present in -sys.modules), this initializes the module; otherwise it simply returns -the value of sys.modules["<modulename>"]. Note that it doesn't enter -the module into any namespace -- it only ensures it has been -initialized and is stored in sys.modules. - -You can then access the module's attributes (i.e. any name defined in -the module) as follows: - - attr = getattr(module, "<attrname>"); - -Calling setattr(), to assign to variables in the module, also works. - -5.10. Q. How do I interface to C++ objects from Python? - -A. Depending on your requirements, there are many approaches. To do -this manually, begin by reading the "Extending and Embedding" document -(Doc/ext.tex, see also <URL:http://www.python.org/doc/>). Realize -that for the Python run-time system, there isn't a whole lot of -difference between C and C++ -- so the strategy to build a new Python -type around a C structure (pointer) type will also work for C++ -objects. - -A useful automated approach (which also works for C) is SWIG: -<URL:http://www.cs.utah.edu/~beazley/SWIG/>. - - -6. Python's design -================== - -6.1. Q. Why isn't there a switch or case statement in Python? - -A. You can do this easily enough with a sequence of -if... elif... elif... else. There have been some proposals for switch -statement syntax, but there is no consensus (yet) on whether and how -to do range tests. - -6.2. Q. Why does Python use indentation for grouping of statements? - -A. Basically I believe that using indentation for grouping is -extremely elegant and contributes a lot to the clarity of the average -Python program. Most people learn to love this feature after a while. -Some arguments for it: - -- Since there are no begin/end brackets there cannot be a disagreement -between grouping perceived by the parser and the human reader. I -remember long ago seeing a C fragment like this: - - if (x <= y) - x++; - y--; - z++; - -and staring a long time at it wondering why y was being decremented -even for x > y... (And I wasn't a C newbie then either.) - -- Since there are no begin/end brackets, Python is much less prone to -coding-style conflicts. In C there are loads of different ways to -place the braces (including the choice whether to place braces around -single statements in certain cases, for consistency). If you're used -to reading (and writing) code that uses one style, you will feel at -least slightly uneasy when reading (or being required to write) -another style. - -- Many coding styles place begin/end brackets on a line by themself. -This makes programs considerably longer and wastes valuable screen -space, making it harder to get a good overview over a program. -Ideally, a function should fit on one basic tty screen (say, 20 -lines). 20 lines of Python are worth a LOT more than 20 lines of C. -This is not solely due to the lack of begin/end brackets (the lack of -declarations also helps, and the powerful operations of course), but -it certainly helps! - -6.3. Q. Why are Python strings immutable? - -A. There are two advantages. One is performance: knowing that a -string is immutable makes it easy to lay it out at construction time --- fixed and unchanging storage requirements. (This is also one of -the reasons for the distinction between tuples and lists.) The -other is that strings in Python are considered as "elemental" as -numbers. No amount of activity will change the value 8 to anything -else, and in Python, no amount of activity will change the string -"eight" to anything else. (Adapted from Jim Roskind) - -6.4. Q. Why don't strings have methods like index() or sort(), like -lists? - -A. Good question. Strings currently don't have methods at all -(likewise tuples and numbers). Long ago, it seemed unnecessary to -implement any of these functions in C, so a standard library module -"string" written in Python was created that performs string related -operations. Since then, the cry for performance has moved most of -them into the built-in module strop (this is imported by module -string, which is still the preferred interface, without loss of -performance except during initialization). Some of these functions -(e.g. index()) could easily be implemented as string methods instead, -but others (e.g. sort()) can't, since their interface prescribes that -they modify the object, while strings are immutable (see the previous -question). - -6.5. Q. Why does Python use methods for some functionality -(e.g. list.index()) but functions for other (e.g. len(list))? - -A. Functions are used for those operations that are generic for a -group of types and which should work even for objects that don't have -methods at all (e.g. numbers, strings, tuples). Also, implementing -len(), max(), min() as a built-in function is actually less code than -implementing them as methods for each type. One can quibble about -individual cases but it's really too late to change such things -fundamentally now. - -6.6. Q. Why can't I derive a class from built-in types (e.g. lists or -files)? - -A. This is caused by the relatively late addition of (user-defined) -classes to the language -- the implementation framework doesn't easily -allow it. See the answer to question 4.2 for a work-around. This -*may* be fixed in the (distant) future. - -6.7. Q. Why must 'self' be declared and used explicitly in method -definitions and calls? - -A. By asking this question you reveal your C++ background. :-) -When I added classes, this was (again) the simplest way of -implementing methods without too many changes to the interpreter. I -borrowed the idea from Modula-3. It turns out to be very useful, for -a variety of reasons. - -First, it makes it more obvious that you are using a method or -instance attribute instead of a local variable. Reading "self.x" or -"self.meth()" makes it absolutely clear that an instance variable or -method is used even if you don't know the class definition by heart. -In C++, you can sort of tell by the lack of a local variable -declaration (assuming globals are rare or easily recognizable) -- but -in Python, there are no local variable declarations, so you'd have to -look up the class definition to be sure. - -Second, it means that no special syntax is necessary if you want to -explicitly reference or call the method from a particular class. In -C++, if you want to use a method from base class that is overridden in -a derived class, you have to use the :: operator -- in Python you can -write baseclass.methodname(self, <argument list>). This is -particularly useful for __init__() methods, and in general in cases -where a derived class method wants to extend the base class method of -the same name and thus has to call the base class method somehow. - -Lastly, for instance variables, it solves a syntactic problem with -assignment: since local variables in Python are (by definition!) those -variables to which a value assigned in a function body (and that -aren't explicitly declared global), there has to be some way to tell -the interpreter that an assignment was meant to assign to an instance -variable instead of to a local variable, and it should preferably be -syntactic (for efficiency reasons). C++ does this through -declarations, but Python doesn't have declarations and it would be a -pity having to introduce them just for this purpose. Using the -explicit "self.var" solves this nicely. Similarly, for using instance -variables, having to write "self.var" means that references to -unqualified names inside a method don't have to search the instance's -directories. - -6.8. Q. Can't you emulate threads in the interpreter instead of -relying on an OS-specific thread implementation? - -A. Unfortunately, the interpreter pushes at least one C stack frame -for each Python stack frame. Also, extensions can call back into -Python at almost random moments. Therefore a complete threads -implementation requires thread support for C. - -6.9. Q. Why can't lambda forms contain statements? - -A. Python lambda forms cannot contain statements because Python's -syntactic framework can't handle statements nested inside expressions. - -However, in Python, this is not a serious problem. Unlike lambda -forms in other languages, where they add functionality, Python lambdas -are only a shorthand notation if you're too lazy to define a function. - -Functions are already first class objects in Python, and can be -declared in a local scope. Therefore the only advantage of using a -lambda form instead of a locally-defined function is that you'll have -to invent a name for the function -- but that's just a local variable -to which the function object (which is exactly the same type of object -that a lambda form yields) is assigned! - -6.10. Q. Why don't lambdas have access to variables defined in the -containing scope? - -A. Because they are implemented as ordinary functions. -See question 4.5 above. - -6.11. Q. Why can't recursive functions be defined inside other functions? - -A. See question 4.5 above. - -6.12. Q. Why is there no more efficient way of iterating over a dictionary -than first constructing the list of keys()? - -A. Have you tried it? I bet it's fast enough for your purposes! In -most cases such a list takes only a few percent of the space occupied -by the dictionary -- it needs only 4 bytes (the size of a pointer) per -key -- a dictionary costs 8 bytes per key plus between 30 and 70 -percent hash table overhead, plus the space for the keys and values -- -by necessity all keys are unique objects and a string object (the most -common key type) costs at least 18 bytes plus the length of the -string. Add to that the values contained in the dictionary, and you -see that 4 bytes more per item really isn't that much more memory... - -A call to dict.keys() makes one fast scan over the dictionary -(internally, the iteration function does exist) copying the pointers -to the key objects into a pre-allocated list object of the right size. -The iteration time isn't lost (since you'll have to iterate anyway -- -unless in the majority of cases your loop terminates very prematurely -(which I doubt since you're getting the keys in random order). - -I don't expose the dictionary iteration operation to Python -programmers because the dictionary shouldn't be modified during the -entire iteration -- if it is, there's a very small chance that the -dictionary is reorganized because the hash table becomes too full, and -then the iteration may miss some items and see others twice. Exactly -because this only occurs rarely, it would lead to hidden bugs in -programs: it's easy never to have it happen during test runs if you -only insert or delete a few items per iteration -- but your users will -surely hit upon it sooner or later. - -6.13. Q. Can Python be compiled to machine code, C or some other language? - -A. Not easily. Python's high level data types, dynamic typing of -objects and run-time invocation of the interpreter (using eval() or -exec) together mean that a "compiled" Python program would probably -consist mostly of calls into the Python run-time system, even for -seemingly simple operations like "x+1". Thus, the performance gain -would probably be minimal. - -Internally, Python source code is always translated into a "virtual -machine code" or "byte code" representation before it is interpreted -(by the "Python virtual machine" or "bytecode interpreter"). In order -to avoid the overhead of parsing and translating modules that rarely -change over and over again, this byte code is written on a file whose -name ends in ".pyc" whenever a module is parsed (from a file whose -name ends in ".py"). When the corresponding .py file is changed, it -is parsed and translated again and the .pyc file is rewritten. There -is no performance difference once the .pyc file has been loaded (the -bytecode read from the .pyc file is exactly the same as the bytecode -created by direct translation). The only difference is that loading -code from a .pyc file is faster than parsing and translating a .py -file, so the presence of precompiled .pyc files will generally improve -start-up time of Python scripts. If desired, the Lib/compileall.py -module/script can be used to force creation of valid .pyc files for a -given set of modules. - -If you are looking for a way to translate Python programs in order to -distribute them in binary form, without the need to distribute the -interpreter and library as well, have a look at the freeze.py script -in the Tools/freeze directory. This creates a single binary file -incorporating your program, the Python interpreter, and those parts of -the Python library that are needed by your program. Of course, the -resulting binary will only run on the same type of platform as that -used to create it. - -Hints for proper usage of freeze.py: - -- the script must be in a file whose name ends in .py - -- you must have installed Python fully: - - make install - make libinstall - make inclinstall - make libainstall - -6.14. Q. Why doesn't Python use proper garbage collection? - -A. It's looking less and less likely that Python will ever get -"automatic" garbage collection (GC). For one thing, unless this were -added to C as a standard feature, it's a portability pain in the ass. -And yes, I know about the Xerox library. It has bits of assembler -code for *most* *common* platforms. Not for all. And although it is -mostly transparent, it isn't completely transparent (when I once -linked Python with it, it dumped core). - -"Proper" GC also becomes a problem when Python gets embedded into -other applications. While in a stand-alone Python it may be fine to -replace the standard malloc() and free() with versions provided by the -GC library, an application embedding Python may want to have its *own* -substitute for malloc() and free(), and may not want Python's. Right -now, Python works with anything that implements malloc() and free() -properly. - -Besides, the predictability of destructor calls in Python is kind of -attractive. With GC, the following code (which is fine in current -Python) will run out of file descriptors long before it runs out of -memory: - - for file in <very long list of files>: - f = open(file) - c = file.read(1) - -Using the current reference counting and destructor scheme, each new -assignment to f closes the previous file. Using GC, this is not -guaranteed. Sure, you can think of ways to fix this. But it's not -off-the-shelf technology. - - -7. Using Python on non-UNIX platforms -===================================== - -7.1. Q. Is there a Mac version of Python? - -A. Yes, see the "mac" subdirectory of the distribution sites, -e.g. <URL:ftp://ftp.python.org/pub/python/mac/>. - -7.2. Q. Are there DOS and Windows versions of Python? - -A. Yes. There is a plethora of not-always-compatible versions. See -the "pythonwin", "wpy", "nt" and "pc" subdirectories of the -distribution sites. A quick comparison: - -PythonWin: Extensive support for the 32-bit native Windows API and GUI -building using MFC. Windows NT and Windows 95 only (and Windows -3.1(1) using win32s, until Microsoft stops supporting it :-( ). -<URL:ftp://ftp.python.org/pub/python/pythonwin/>. - -WPY: Ports to DOS, Windows 3.1(1), Windows 95, Windows NT and OS/2. -Also contains a GUI package that offers portability between Windows -(not DOS) and Unix, and native look and feel on both. -<URL:ftp://ftp.python.org/pub/python/wpy/>. - -NT: Basic ports built straight from the 1.4 distribution for Windows -95 and Windows NT. This will eventually provide core support for -both PythonWin and WPY on all 32-bit Microsoft platforms. -<URL:ftp://ftp.python.org/pub/python/nt/>. - -PC: Old, unsupported ports to DOS, Windows 3.1(1) and OS/2. -<URL:ftp://ftp.python.org/pub/python/pc/>. - -7.3. Q. Is there an OS/2 version of Python? - -A. Yes, see the "pc" and "wpy" subdirectory of the distribution sites -(see above). - -7.4. Q. Is there a VMS version of Python? - -A. Donn Cave <donn@cac.washington.edu> did a partial port. The -results of his efforts are on public display in -<<URL:ftp://ftp.python.org/pub/python/contrib/vms.tar.gz/>. Someone -else is working on a more complete port, for details watch the list. - -7.5. Q. What about IBM mainframes, or other non-UNIX platforms? - -A. I haven't heard about these, except I remember hearing about an -OS/9 port and a port to Vxworks (both operating systems for embedded -systems). If you're interested in any of this, go directly to the -newsgroup and ask there, you may find exactly what you need. For -example, a port to MPE/iX 5.0 on HP3000 computers was just announced, -see <URL:http://www.allegro.com/software/>. - -7.6. Q. Where are the source or Makefiles for the non-UNIX versions? - -A. The standard sources can (almost) be used. Additional sources can -be found in the platform-specific subdirectories of the distribution. - -7.7. Q. What is the status and support for the non-UNIX versions? - -A. I don't have access to most of these platforms, so in general I am -dependent on material submitted by volunteers(*). However I strive to -integrate all changes needed to get it to compile on a particular -platform back into the standard sources, so porting of the next -version to the various non-UNIX platforms should be easy. - -(*) For the Macintosh, that volunteer is me, with help from Jack -Jansen <jack@cwi.nl>. - -7.8. Q. I have a PC version but it appears to be only a binary. -Where's the library? - -A. You still need to copy the files from the distribution directory -"python/Lib" to your system. If you don't have the full distribution, -you can get the file lib<version>.tar.gz from most ftp sites carrying -Python; this is a subset of the distribution containing just those -files, e.g. <URL:ftp://ftp.python.org/pub/python/src/lib1.1.tar.gz>. - -Once you have installed the library, you need to point sys.path to it. -Assuming the library is in C:\misc\python\lib, the following commands -will point your Python interpreter to it (note the doubled backslashes --- you can also use single forward slashes instead): - - >>> import sys - >>> sys.path.insert(0, 'C:\\misc\\python\\lib') - >>> - -For a more permanent effect, set the environment variable PYTHONPATH, -as follows (talking to a DOS prompt): - - C> SET PYTHONPATH=C:\misc\python\lib - -7.9. Q. Where's the documentation for the Mac or PC version? - -A. The documentation for the Unix version also applies to the Mac and -PC versions. Where applicable, differences are indicated in the text. - -7.10. Q. The Mac (PC) version doesn't seem to have any facilities for -creating or editing programs apart from entering it interactively, and -there seems to be no way to save code that was entered interactively. -How do I create a Python program on the Mac (PC)? - -A. Use an external editor. On the Mac, BBEdit seems to be a popular -no-frills text editor. I work like this: start the interpreter; edit -a module file using BBedit; import and test it in the interpreter; -edit again in BBedit; then use the built-in function reload() to -re-read the imported module; etc. - -Regarding the same question for the PC, Kurt Wm. Hemr writes: "While -anyone with a pulse could certainly figure out how to do the same on -MS-Windows, I would recommend the NotGNU Emacs clone for MS-Windows. -Not only can you easily resave and "reload()" from Python after making -changes, but since WinNot auto-copies to the clipboard any text you -select, you can simply select the entire procedure (function) which -you changed in WinNot, switch to QWPython, and shift-ins to reenter -the changed program unit." diff --git a/Misc/README b/Misc/README index 0ad373b..c737c5e 100644 --- a/Misc/README +++ b/Misc/README @@ -14,7 +14,7 @@ AIX-NOTES Notes for building Python on AIX (all new!) BLURB A quick description of Python for newcomers BLURB.LUTZ A testimonial from a converted Tcl/Perl hacker :-) COPYRIGHT The Python copyright notice -FAQ Frequently Asked Questions about Python (and answers) +FAQ (deleted -- see http://www.python.org for the online FAQ) Fixcprt.py Fix the copyright message (a yearly chore :-) HISTORY News from previous releases -- oldest last HPUX-NOTES Notes about dynamic loading under HP-UX |