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-rw-r--r--Misc/FAQ186
1 files changed, 157 insertions, 29 deletions
diff --git a/Misc/FAQ b/Misc/FAQ
index 72a85c4..cbbb954 100644
--- a/Misc/FAQ
+++ b/Misc/FAQ
@@ -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
===================