Some updates. Still out of date.
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Guido van Rossum
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Misc/FAQ
186
Misc/FAQ
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@ -9,8 +9,8 @@ Approved: news-answers-request@MIT.Edu
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Archive-name: python-faq/part1
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Submitted-by: Guido van Rossum <guido@cwi.nl>
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Version: 1.29
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Last-modified: 11 October 1995
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Version: 1.31
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Last-modified: 26 June 1996
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This article contains answers to Frequently Asked Questions about
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Python (an object-oriented interpreted programming language -- see
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@ -29,7 +29,7 @@ Author's address:
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Email: <guido@python.org>, <guido@cnri.reston.va.us>, <guido@cwi.nl>
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The latest version of this FAQ is available by anonymous ftp from
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<URL:ftp://ftp.python.org/pub/doc/FAQ>. It will also be posted
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<URL:ftp://ftp.python.org/pub/python/doc/FAQ>. It will also be posted
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regularly to the newsgroups comp.answers <URL:news:comp.answers> and
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comp.lang.python <URL:news:comp.lang.python>.
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@ -106,16 +106,20 @@ Here's an overview of the questions per chapter:
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3.7. Q. I use VPATH but some targets are built in the source directory.
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3.8. Q. Trouble building or linking with the GNU readline library.
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3.9. Q. Trouble building Python on Linux.
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3.10. Q. Trouble with prototypes on Ultrix.
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3.11. Q. Trouble with posix.listdir on NeXTSTEP 3.2.
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3.12. Q. Other trouble building Python on platform X.
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3.13. Q. How to configure dynamic loading on Lixux.
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3.14. Q. Under Solaris 2.x, using GCC, how do I use shared libraries?
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3.15. Q. Errors when linking with a shared library containing C++ code.
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3.16. Q. Tk menus or radiobuttons don't work properly in Python 1.2.
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3.17. Q. I built with _tkintermodule.c enabled but get "Tkinter not found".
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3.18. Q. I built with Tk 4.0 but Tkinter complains about the Tk version.
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3.19. Q. Tk doesn't work right on DEC Alpha.
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3.10. Q. Trouble with socket I/O on Linux.
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3.11. Q. Trouble with prototypes on Ultrix.
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3.12. Q. Trouble with posix.listdir on NeXTSTEP 3.2.
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3.13. Q. Other trouble building Python on platform X.
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3.14. Q. How to configure dynamic loading on Linux.
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3.15. Q. Under Solaris 2.x, using GCC, how do I use shared libraries?
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3.16. Q. Errors when linking with a shared library containing C++ code.
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3.17. Q. Tk menus or radiobuttons don't work properly in Python 1.2.
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3.18. Q. I built with tkintermodule.c enabled but get "Tkinter not found".
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3.19. Q. I built with Tk 4.0 but Tkinter complains about the Tk version.
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3.20. Q. Tk doesn't work right on DEC Alpha.
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3.21. Q. Several common system calls are missing from the posix module.
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3.22. Q. ImportError: No module named string, on MS Windows.
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3.23. Q. Core dump on SGI when using the gl module.
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4. Programming in Python
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4.1. Q. Is there a source code level debugger with breakpoints, step,
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@ -169,6 +173,8 @@ Here's an overview of the questions per chapter:
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4.31. Q. How do I call a function if I have the arguments in a tuple?
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4.32. Q. How do I enable font-lock-mode for Python in Emacs?
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4.33. Q. Is there an inverse to the format operator (a la C's scanf())?
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4.34. Q. Can I have Tk events handled while waiting for I/O?
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4.35. Q. How do I write a function with output parameters (call by reference)?
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5. Extending Python
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5.1. Q. Can I create my own functions in C?
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@ -788,12 +794,18 @@ from Lib/test/test_b2.py.
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Apart from this, Python builds and runs fine on most Linux versions
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(if you run into trouble on an old Linux version, consider upgrading).
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3.10. Q. Trouble with prototypes on Ultrix.
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3.10. Q. Trouble with socket I/O on Linux.
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A. Once you've built Python, use it to run the regen.py script in the
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Lib/linux1 directory. Apparently the files as distributed don't match
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the system headers on some Linux versions.
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3.11. Q. Trouble with prototypes on Ultrix.
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A. Ultrix cc seems broken -- use gcc, or edit config.h to #undef
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HAVE_PROTOTYPES.
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3.11. Q. Trouble with posix.listdir on NeXTSTEP 3.2.
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3.12. Q. Trouble with posix.listdir on NeXTSTEP 3.2.
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A. (This often manifests itself as a weird error from the
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compileall.py script run by "make libinstall".)
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@ -803,7 +815,7 @@ derived from (an old version of) gcc, its interpretation of the
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"-posix" switch is different; in this particular case, cc is right and
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gcc is wrong.
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3.12. Q. Other trouble building Python on platform X.
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3.13. Q. Other trouble building Python on platform X.
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A. Please email the details to <guido@cwi.nl> and I'll look into it.
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Please provide as many details as possible. In particular, if you
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@ -812,7 +824,7 @@ version) you are using it will be difficult for me to figure out what
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is the matter. If you get a specific error message, please email it
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to me too.
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3.13. Q. How to configure dynamic loading on Lixux.
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3.14. Q. How to configure dynamic loading on Linux.
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A. There are two answers, depending on whether you are using the new
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ELF object format or not.
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@ -860,23 +872,23 @@ of using dynamic libraries on Linux are:
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4) Recompile Python.
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3.14. Q. Under Solaris 2.x, using GCC, how do I use shared libraries?
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3.15. Q. Under Solaris 2.x, using GCC, how do I use shared libraries?
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A. Use the linker in /usr/ucb/ld, not the GNU linker. The latter
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cannot create shared libraries.
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3.15. Q. Errors when linking with a shared library containing C++ code.
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3.16. Q. Errors when linking with a shared library containing C++ code.
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A. Link the main Python binary with C++. Change the definition of
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LINKCC in Modules/Makefile to be your C++ compiler. You may have to
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edit config.c slightly to make it compilable with C++.
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3.16. Q. Tk menus or radiobuttons don't work properly in Python 1.2.
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3.17. Q. Tk menus or radiobuttons don't work properly in Python 1.2.
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A. There's a bug in Tkinter.py; remove ClassType and InstanceType from
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the definition of CallableTypes near the top of Lib/tkinter/Tkinter.py.
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3.17. Q. I built with _tkintermodule.c enabled but get "Tkinter not found".
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3.18. Q. I built with tkintermodule.c enabled but get "Tkinter not found".
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A. Tkinter.py (note: upper case T) lives in a subdirectory of Lib,
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Lib/tkinter. If you are using the default module search path, you
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@ -884,7 +896,7 @@ probably didn't enable the line in the Modules/Setup file defining
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TKPATH; if you use the environment variable PYTHONPATH, you'll have to
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add the proper tkinter subdirectory.
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3.18. Q. I built with Tk 4.0 but Tkinter complains about the Tk version.
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3.19. Q. I built with Tk 4.0 but Tkinter complains about the Tk version.
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A. Several things could cause this. You most likely have a Tk 3.6
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installation that wasn't completely eradicated by the Tk 4.0
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@ -895,7 +907,7 @@ have compiled Python with the old tk.h header file (yes, this actually
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compiles!); you may actually have linked with Tk 3.6 even though Tk
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4.0 is also around. Similar for Tcl 7.4 vs. Tcl 7.3.
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3.19. Q. Tk doesn't work right on DEC Alpha.
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3.20. Q. Tk doesn't work right on DEC Alpha.
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A. You probably compiled either Tcl, Tk or Python with gcc. Don't.
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For this platform, which has 64-bit integers, gcc is known to generate
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@ -907,6 +919,33 @@ far as we know, there are no problem with gcc on other platforms --
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the instabilities seem to be restricted to the DEC Alpha.) See also
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question 3.6.
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3.21. Q. Several common system calls are missing from the posix module.
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A. Most likely, *all* test compilations run by the configure script
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are failing for some reason or another. Have a look in config.log to
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see what could be the reason. A common reason is specifying a
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directory to the --with-readline option that doesn't contain the
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libreadline.a file.
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3.22. Q. ImportError: No module named string, on MS Windows.
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A. Most likely, your PYTHONPATH environment variable should be set to
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something like:
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set PYTHONPATH=c:\python;c:\python\lib;c:\python\scripts
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(assuming Python was installed in c:\python)
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3.23. Q. Core dump on SGI when using the gl module.
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There are conflicts between entry points in the termcap and curses
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libraries and an entry point in the GL library. There's a hack of a
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fix for the termcap library if it's needed for the GNU readline
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library, but it doesn't work when you're using curses. Concluding,
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you can't build a Python binary containing both the curses and gl
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modules.
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4. Programming in Python
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========================
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@ -1111,10 +1150,10 @@ the module only works on specific platforms (e.g. gl only works on SGI
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machines).
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NOTE: if the complaint is about "Tkinter" (upper case T) and you have
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already configured module "_tkinter" (lower case t and underscore),
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the solution is *not* to rename tkinter to Tkinter or vice versa.
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There is probably something wrong with your module search path. Check
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out the value of sys.path.
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already configured module "tkinter" (lower case t), the solution is
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*not* to rename tkinter to Tkinter or vice versa. There is probably
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something wrong with your module search path. Check out the value of
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sys.path.
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For X-related modules (Xt and Xm) you will have to do more work: they
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are currently not part of the standard Python distribution. You will
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@ -1498,9 +1537,9 @@ due to Jack Jansen:
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# Child
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os.close(0)
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os.close(1)
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if os.dup(p2cread) <> 0:
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if os.dup(p2cread) != 0:
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sys.stderr.write('popen2: bad read dup\n')
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if os.dup(c2pwrite) <> 1:
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if os.dup(c2pwrite) != 1:
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sys.stderr.write('popen2: bad write dup\n')
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for i in range(3, MAXFD):
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try:
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@ -1568,6 +1607,95 @@ string.strip() which removes surrounding whitespace from a string).
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For more complicated input parsing, regular expressions (see module
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regex) are better suited and more powerful than C's scanf().
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4.34. Q. Can I have Tk events handled while waiting for I/O?
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A. Yes, and you don't even need threads! But you'll have to
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restructure your I/O code a bit. Tk has the equivalent of Xt's
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XtAddInput() call, which allows you to register a callback function
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which will be called from the Tk mainloop when I/O is possible on a
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file descriptor. Here's what you need:
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from Tkinter import tkinter
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tkinter.createfilehandler(file, mask, callback)
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The file may be a Python file or socket object (actually, anything
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with a fileno() method), or an integer file descriptor. The mask is
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one of the constants tkinter.READABLE or tkinter.WRITABLE. The
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callback is called as follows:
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callback(file, mask)
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You must unregister the callback when you're done, using
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tkinter.deletefilehandler(file)
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Note: since you don't know *how many bytes* are available for reading,
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you can't use the Python file object's read or readline methods, since
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these will insist on reading a predefined number of bytes. For
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sockets, the recv() or recvfrom() methods will work fine; for other
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files, use os.read(file.fileno(), maxbytecount).
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4.35. Q. How do I write a function with output parameters (call by reference)?
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A. [Mark Lutz] The thing to remember is that arguments are passed by
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assignment in Python. Since assignment just creates references to
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objects, there's no alias between an argument name in the caller and
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callee, and so no call-by-reference per se. But you can simulate it
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in a number of ways:
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1) By using global variables; but you probably shouldn't :-)
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2) By passing a mutable (changeable in-place) object:
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def func1(a):
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a[0] = 'new-value' # 'a' references a mutable list
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a[1] = a[1] + 1 # changes a shared object
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args = ['old-value', 99]
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func1(args)
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print args[0], args[1] # output: new-value 100
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3) By return a tuple, holding the final values of arguments:
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def func2(a, b):
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a = 'new-value' # a and b are local names
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b = b + 1 # assigned to new objects
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return a, b # return new values
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x, y = 'old-value', 99
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x, y = func2(x, y)
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print x, y # output: new-value 100
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4) And other ideas that fall-out from Python's object model. For
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instance, it might be clearer to pass in a mutable dictionary:
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def func3(args):
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args['a'] = 'new-value' # args is a mutable dictionary
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args['b'] = args['b'] + 1 # change it in-place
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args = {'a':' old-value', 'b': 99}
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func3(args)
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print args['a'], args['b']
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5) Or bundle-up values in a class instance:
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class callByRef:
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def __init__(self, **args):
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for (key, value) in args.items():
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setattr(self, key, value)
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def func4(args):
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args.a = 'new-value' # args is a mutable callByRef
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args.b = args.b + 1 # change object in-place
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args = callByRef(a='old-value', b=99)
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func4(args)
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print args.a, args.b
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But there's probably no good reason to get this complicated :-).
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[Python' author favors solution 3 in most cases.]
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5. Extending Python
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===================
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