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@@ -9,8 +9,8 @@ Approved: news-answers-request@MIT.Edu Archive-name: python-faq/part1 Submitted-by: Guido van Rossum <guido@cwi.nl> -Version: 1.29 -Last-modified: 11 October 1995 +Version: 1.31 +Last-modified: 26 June 1996 This article contains answers to Frequently Asked Questions about Python (an object-oriented interpreted programming language -- see @@ -29,7 +29,7 @@ Author's address: Email: <guido@python.org>, <guido@cnri.reston.va.us>, <guido@cwi.nl> The latest version of this FAQ is available by anonymous ftp from -<URL:ftp://ftp.python.org/pub/doc/FAQ>. It will also be posted +<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>. @@ -106,16 +106,20 @@ Here's an overview of the questions per chapter: 3.7. Q. I use VPATH but some targets are built in the source directory. 3.8. Q. Trouble building or linking with the GNU readline library. 3.9. Q. Trouble building Python on Linux. - 3.10. Q. Trouble with prototypes on Ultrix. - 3.11. Q. Trouble with posix.listdir on NeXTSTEP 3.2. - 3.12. Q. Other trouble building Python on platform X. - 3.13. Q. How to configure dynamic loading on Lixux. - 3.14. Q. Under Solaris 2.x, using GCC, how do I use shared libraries? - 3.15. Q. Errors when linking with a shared library containing C++ code. - 3.16. Q. Tk menus or radiobuttons don't work properly in Python 1.2. - 3.17. Q. I built with _tkintermodule.c enabled but get "Tkinter not found". - 3.18. Q. I built with Tk 4.0 but Tkinter complains about the Tk version. - 3.19. Q. Tk doesn't work right on DEC Alpha. + 3.10. Q. Trouble with socket I/O on Linux. + 3.11. Q. Trouble with prototypes on Ultrix. + 3.12. Q. Trouble with posix.listdir on NeXTSTEP 3.2. + 3.13. Q. Other trouble building Python on platform X. + 3.14. Q. How to configure dynamic loading on Linux. + 3.15. Q. Under Solaris 2.x, using GCC, how do I use shared libraries? + 3.16. Q. Errors when linking with a shared library containing C++ code. + 3.17. Q. Tk menus or radiobuttons don't work properly in Python 1.2. + 3.18. Q. I built with tkintermodule.c enabled but get "Tkinter not found". + 3.19. Q. I built with Tk 4.0 but Tkinter complains about the Tk version. + 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, @@ -169,6 +173,8 @@ Here's an overview of the questions per chapter: 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)? 5. Extending Python 5.1. Q. Can I create my own functions in C? @@ -788,12 +794,18 @@ from Lib/test/test_b2.py. Apart from this, Python builds and runs fine on most Linux versions (if you run into trouble on an old Linux version, consider upgrading). -3.10. Q. Trouble with prototypes on Ultrix. +3.10. Q. Trouble with socket I/O on Linux. + +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. + +3.11. Q. Trouble with prototypes on Ultrix. A. Ultrix cc seems broken -- use gcc, or edit config.h to #undef HAVE_PROTOTYPES. -3.11. Q. Trouble with posix.listdir on NeXTSTEP 3.2. +3.12. Q. Trouble with posix.listdir on NeXTSTEP 3.2. A. (This often manifests itself as a weird error from the compileall.py script run by "make libinstall".) @@ -803,7 +815,7 @@ derived from (an old version of) gcc, its interpretation of the "-posix" switch is different; in this particular case, cc is right and gcc is wrong. -3.12. Q. Other trouble building Python on platform X. +3.13. Q. Other trouble building Python on platform X. A. Please email the details to <guido@cwi.nl> and I'll look into it. Please provide as many details as possible. In particular, if you @@ -812,7 +824,7 @@ 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. -3.13. Q. How to configure dynamic loading on Lixux. +3.14. Q. How to configure dynamic loading on Linux. A. There are two answers, depending on whether you are using the new ELF object format or not. @@ -860,23 +872,23 @@ of using dynamic libraries on Linux are: 4) Recompile Python. -3.14. Q. Under Solaris 2.x, using GCC, how do I use shared libraries? +3.15. Q. Under Solaris 2.x, using GCC, how do I use shared libraries? A. Use the linker in /usr/ucb/ld, not the GNU linker. The latter cannot create shared libraries. -3.15. Q. Errors when linking with a shared library containing C++ code. +3.16. 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. Tk menus or radiobuttons don't work properly in Python 1.2. +3.17. Q. Tk menus or radiobuttons don't work properly in Python 1.2. A. There's a bug in Tkinter.py; remove ClassType and InstanceType from the definition of CallableTypes near the top of Lib/tkinter/Tkinter.py. -3.17. Q. I built with _tkintermodule.c enabled but get "Tkinter not found". +3.18. 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 @@ -884,7 +896,7 @@ 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. -3.18. Q. I built with Tk 4.0 but Tkinter complains about the Tk version. +3.19. 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 @@ -895,7 +907,7 @@ 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. -3.19. Q. Tk doesn't work right on DEC Alpha. +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 @@ -907,6 +919,33 @@ 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 ======================== @@ -1111,10 +1150,10 @@ 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 and underscore), -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. +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 @@ -1498,9 +1537,9 @@ due to Jack Jansen: # Child os.close(0) os.close(1) - if os.dup(p2cread) <> 0: + if os.dup(p2cread) != 0: sys.stderr.write('popen2: bad read dup\n') - if os.dup(c2pwrite) <> 1: + if os.dup(c2pwrite) != 1: sys.stderr.write('popen2: bad write dup\n') for i in range(3, MAXFD): try: @@ -1568,6 +1607,95 @@ 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.] + 5. Extending Python =================== |