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diff --git a/Misc/FAQ b/Misc/FAQ new file mode 100644 index 0000000..49b1607 --- /dev/null +++ b/Misc/FAQ @@ -0,0 +1,643 @@ +Subject: FAQ: Python -- an object-oriented language +Newsgroups: comp.lang.misc,comp.answers,news.answers +Followup-to: comp.lang.misc +From: guido@cwi.nl (Guido van Rossum) +Reply-to: guido@cwi.nl (Guido van Rossum) +Approved: news-answers-request@MIT.Edu + +Archive-name: python-faq/part1 +Version: 1.2 +Last-modified: 24 Jan 1994 + +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, 1994 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 + CWI, dept. CST + Kruislaan 413 + P.O. Box 94079 + 1090 GB Amsterdam + The Netherlands +Email: guido@cwi.nl + +The latest version of this FAQ is available by anonymous ftp from +ftp.cwi.nl [192.16.184.180] in the directory /pub/python, with +filename python-FAQ. It will also be posted regularly to the +newsgroups comp.answers and comp.lang.misc. + +Many FAQs, including this one, are available by anonymous ftp from +rtfm.mit.edu [18.70.0.209] in the directory 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. + +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 + 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. Is there a newsgroup or mailing list devoted to Python? + 1.6. Q. Is there a book on Python, or will there be one out soon? + 1.7. Q. Are there any published articles about Python that I can quote? + + 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. What new developments are expected for Python in the future? + 2.5. Q. How stable is Python? + 2.6. Q. Any more future plans? + + 3. Building Python + 3.1. Q. I have trouble building the md5 module and/or finding the file + md5.c. + 3.2. Q. Is there a test set? + 3.3. 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. + 3.4. Q. I get an OverflowError on evaluating 2*2. What is going on? + 3.5. Q. Trouble building Python 0.9.9 on platform X. + + 4. Programming in Python + 4.1. 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?) + 4.2. Q. I assign to a variable in a call to exec() but when I try to + use it on the next line I get an error. What is going on? + 4.3. Q. Why does that work? + 4.4. Q. Is there a curses/termcap package for Python? + 4.5. Q. Is there an equivalent to C's onexit() in Python? + 4.6. 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? + 4.7. Q. How do I iterate over a sequence in reverse order? + 4.8. Q. My program is too slow. How do I speed it up? + 4.9. 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? + + 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++? + + 6. Python's design + 6.1. Q. Why isn't there a generic copying operation for objects in + Python? + 6.2. Q. Why isn't there a generic way to implement persistent objects + in Python? (Persistent == automatically saved to and restored from + disk.) + 6.3. Q. Why isn't there a switch or case statement in Python? + + 7. Using Python on non-UNIX platforms + 7.1. Q. Where's the DOS version of 0.9.9? + 7.2. Q. Is there a Windows version of Python? + 7.3. Q. I have the Mac or DOS version but it appears to be only a binary. + Where's the library? + 7.4. Q. Where's the documentation for the Mac or DOS version? + 7.5. Q. The Mac 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? + +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 +MS-DOS. + +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 wizard, I'm also a fan of "Monty +Python's Flying Circus" (a BBC comedy series from the seventies, in +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! + +1.3. Q. How do I obtain a copy of the Python source? + +A. The latest Python source distribution is always available by +anonymous ftp from ftp.cwi.nl [192.16.184.180] in the directory +/pub/python, with filename python<version>.tar.Z. It is a compressed +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. Currently <version> is 0.9.9. (See section 7 +for non-UNIX information.) + +1.4. Q. How do I get documentation on Python? + +A. The latest Python documentation set is always available by +anonymous ftp from ftp.cwi.nl [192.16.184.180] in the directory +/pub/python, with filename pythondoc-ps<version>.tar.Z. It is a +compressed tar file containing PostScript files of the reference +manual, the library manual, and the tutorial. Currently <version> is +0.9.9. (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. + +The following sites keep mirrors of the Python distribution: + +Site IP address Directory + +gatekeeper.dec.com 16.1.0.2 /pub/plan/python/cwi +ftp.uu.net 192.48.96.9 /languages/python +ftp.wustl.edu 128.252.135.4 /graphics/graphics/sgi-stuff/python +ftp.funet.fi 128.214.6.100 /pub/languages/python (old?) +ftp.fu-berlin.de 130.133.4.50 /pub/unix/languages/python (python* only) + +Or try archie on e.g. python0.9.9.tar.Z to locate the nearest copy of +that version... + +1.5. Q. Is there a newsgroup or mailing list devoted to Python? + +A. There is no Python newsgroup yet; if you want to post to the net +about Python, use comp.lang.misc. There is a mailing list devoted to +Python; send e-mail to python-list-request@cwi.nl to (un)subscribe. +There are plans to start the discussion about creation of +comp.lang.python as soon as version 1.0.0 has been released. + +1.6. Q. Is there a book on Python, or will there be one out soon? + +A. Unfortunately, not yet. I would like to write one but my +obligations at CWI include too much other work to make much progress +on it. Several parties have expressed interest in sponsoring or +helping the production of a book or reference manual, but so far there +are no firm plans. If you volunteer help, by all means drop me a +note! + +1.7. 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. + +A more recent high-level description of Python 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 can be found in the ftp directory mentioned a few +questions earlier, with filenames nluug-paper.ps and nluug-slides.ps, +respectively. + + +2. Python in the real world +=========================== + +2.1. Q. How many people are using Python? + +A. I don't know, but at the last count there were at least 130 +addresses on the Python mailing list (several of which are local +redistribution lists). I suspect that many users don't bother +to subscribe to the list. + +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 +multimedia teleconferencing tool, as well as many smaller programs. + +The University of Virginia uses Python to control a virtual reality +engine. Contact: Matt Conway <conway@virginia.edu>. + +See also the next question. + +2.3. Q. Are there any commercial projects going on using Python? + +A. Several companies have revealed to me that they are planning or +considering to use Python in a future product. The furthest is +Sunrise Software, who already have a product out using Python -- they +use Python for a GUI management application and an SNMP network +manangement application. Contact: <info@sunrise.com>. + +Individuals at many other companies are using Python for +internal development (witness their contributions to the Python +mailing list). + +Python has also been elected as an extension language by MADE, a +consortium supported by the European Committee's ESPRIT program and +consisting of Bull, CWI and some other European companies. Contact: +Ivan Herman <ivan@cwi.nl>. + +2.4. Q. What new developments are expected for Python in the future? + +A. I am almost ready to release version 1.0.0 -- it should be out by +the end of January 1994. It will have some new functionality and +bugfixes and be portable to more platforms. The directory tree +structure and build procedure will be radically different -- almost +all configuration is now done automatically, using GNU autoconf. +User-visible changes include: double-quoted strings, functional +programming operations (lambda, map, filter, reduce -- all evaluated +eagerly), exec becomes a statement, str() is customizable through +__str__ (used by print). The originally planned grand renaming scheme +will not be implemented because of lack of time. A beta version can +be ftp'ed from the usual sites, file python1.0.0beta.tar.Z. + +2.5. Q. How stable is Python? + +A. Very stable. While the current version number (0.9.9) would +suggest it is in the early stages of development, in fact new, stable +releases have been coming out every 3-6 months for the past three years. + +2.6. Q. Any more future plans? + +A. Without warranty that any of this will actually be realized: I am +currently thinking about mechanisms for built-in on-line help and a +switch/case statement. There are also some people (independently) +working on a windowing interface based on STDWIN but with the power +and ease of use of the average modern widget set. I still hope to get +some help in producing a Windows version. It would be nice if there +were a window-based class browser (Someone at CWI has contributed one +using Motif but it needs some work). + + +3. Building Python +================== + +3.1. Q. I have trouble building the md5 module and/or finding the file +md5.c. + +A. Apparently the md5 module was based on an older version of RSA's +md5 implementation. The ftp site rsa.com mentioned in the Makefile +where this version was found is no longer accessible, and the version +from RFC 1321 (md5c.c) is slightly different. This will be fixed in +the 1.0 release; write me if you need the fixes now. + +3.2. 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. + +3.3. 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. + +3.4. Q. I get an OverflowError on evaluating 2*2. What is going on? + +A. Your machine probably has 64 bit long integers (e.g. DEC alpha or +HP snake architectures). There are some dependencies on word length +in file intobject.c. This will be corrected in the 1.0 release; until +then, on a 64 bit machine, just comment out the check for overflow +from int_mul: + + #if 0 + if (x > 0x7fffffff || x < (double) (long) 0x80000000) + return err_ovf("integer multiplication"); + #endif + +You should also include <limits.h> and replace the constant 32 by +LONG_BIT in int_[lr]shift. + +3.5. Q. Trouble building Python 0.9.9 on platform X. + +A. In the bootstrap phase (before you have built the first running +interpreter), make sure the -D settings in the Makefile are correct +for your system. In particular you may have to add or delete -DSYSV. +It may also be necessary to change the flags used to compile +posixmodule.c and timemodule.c; e.g. on AIX the following are +necessary: + posixmodule.c: -DHAVE_STDLIB -DNOALTTZ -DOLDTZ -Dunix -DSYSV -DDO_TIMES + timemodule.c: -DHAVE_STDLIB -DNOALTTZ -DOLDTZ -Uunix -DSYSV -DBSD_TIME +(Note the -Uunix for timemodule!) +Those switches for timemodule also require that the + #ifdef unix + #ifdef BSD_TIME +just above: + static long + millitimer() +( and below the "#endif /* macintosh */" version of millitimer +be changed to: + #if defined(unix) | defined(BSD_TIME) + #ifdef BSD_TIME + + +4. Programming in Python +======================== + +4.1. 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) + +4.2. Q. I assign to a variable in a call to exec() but when I try to +use it on the next line I get an error. What is going on? + +A. The reason why this occurs is too complicated to explain (but see +the next question). To fix it is easy, however: simply assign None to +the variable *before* calling exec(). This will be fixed in the 1.0 +release. + +4.3. Q. Why does that work? + +A. When parsing your program and converting it into internal pseudo +code, the interpreter does some optimizations to speed up function +execution: it figures out the names of all the local variables and +treats them specially. Because your assignment is done by exec(), it +is not seen initially by the parser and the variable is not recognized +as a local variable. The default treatment is as a global variable, +but the exec() statement places it in the local scope, where it is not +found. This will be fixed in release 1.0 by making exec into a +statement; the parser will then be able to switch off the +optimizations for local variables if it encounters an exec statement +(recognizing calls to built-in functions is not possible for the +parser, hence the syntax change to a statement). + +4.4. Q. Is there a curses/termcap package for Python? + +A. No, but you can use the "alfa" (== character cell) version of +STDWIN. (STDWIN == Standard Windows, a portable windowing system +interface by the same author, URL ftp://ftp.cwi.nl/pub/stdwin.) +This will also prepare your program for porting to windowing +environments such as X11 or the Macintosh. + +4.5. 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. + +4.6. 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 + +4.7. 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 the the method +call overhead... + +4.8. 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). + +4.9. 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 efficiency reasons, 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 a changed module +being read again, 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 objects imported +thus. + + +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 all explained +in the file "python/misc/EXTENDING". Also read the file "DYNLOAD" +there for hints on how to load such extension modules + +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. + + +6. Python's design +================== + +6.1. Q. Why isn't there a generic copying operation for objects in +Python? + +A. Hmm. Maybe there should be one, but it's difficult to assign a +useful meaning to copying of open files, sockets and windows, or +recursive data structures. As long as you design all your classes +yourself you are of course free to define a standard base class that +defines an overridable copying operation for all the objects you care +about. (One practical point: it would have to be a built-in function, +not a standard method name, since not all built-in object types have +methods; e.g. strings, integers and tuples don't.) + +6.2. Q. Why isn't there a generic way to implement persistent objects +in Python? (Persistent == automatically saved to and restored from +disk.) + +A. Hmm, hmm. Basically for the same reasons as why there is no +generic copying operation. + +6.3. 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 concensus (yet) on whether and how +to do range tests. + + +7. Using Python on non-UNIX platforms +===================================== + +7.1. Q. Where's the DOS version of 0.9.9? + +A. I hope it will be coming soon. A friend with a DOS machine and a +compiler has volunteered to build it but he's very busy. Until then, +you will have to make do with the 0.9.8 version (which isn't so bad, +actually). + +7.2. Q. Is there a Windows version of Python? + +A. Not yet. Several Windows hackers with C compilers are working on a +port though, so maybe we'll have one soon. + +7.3. Q. I have the Mac or DOS 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 ftp the file pythonlib0.9.9.tar.Z from site ftp.cwi.nl, +directory /pub/python; this is a subset of the distribution containing +just those file. + +7.4. Q. Where's the documentation for the Mac or DOS version? + +A. There isn't any. The documentation for the Unix version also +applies to the Mac and DOS versions. Where applicable, differences +are indicated in the text. + +7.5. Q. The Mac 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? + +A. Use an external editor. I am quite happy with the Desk Accessory +called Sigma Edit; this doesn't require Multifinder or System 7. I +work like this: start the interpreter; edit a module file using Sigma +Edit; import and test it in the interpreter; edit again in Sigma Edit; +then use the built-in function reload() to re-read the imported +module; etc. |