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 1993-1995 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
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Skip Montanaro <skip@automatrix.com> maintains an HTML version of this
FAQ, <URL:http://www.automatrix.com/~skip/python-faq.html>.
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. 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 there a book on Python, or will there be one out soon?
1.9. Q. Are there any published articles about Python that I can quote?
1.10. Q. Are there short introductory papers or talks on Python?
1.11. Q. How does the Python version numbering scheme work?
1.12. Q. Are there other ftp sites that carry Python related material?
1.13. Q. Are there copyright restrictions on the use of Python?
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. When will the next version be released?
2.6. Q. What new developments are expected for Python in the future?
2.7. Q. Is it reasonable to propose incompatible changes to Python?
3. Building Python
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 building Python with STDWIN 0.9.8. on SGI IRIX.
3.4. Q. Link errors building Python with STDWIN 0.9.9.
3.5. Q. Link errors after rerunning the configure script.
3.6. Q. The python interpreter complains about options passed to a
script (after the script name).
3.7. Q. When building on the SGI, make tries to run python to create
glmodule.c, but python hasn't been built or installed yet.
3.8. Q. Python built with gcc for the DEC Alpha doesn't work.
3.9. Q. I use VPATH but some targets are built in the source directory.
3.10. Q. Trouble building or linking with the GNU readline library.
3.11. Q. Trouble building Python on Linux.
3.12. Q. Trouble with prototypes on Ultrix.
3.13. Q. Trouble with posix.listdir on NeXTSTEP 3.2.
3.14. Q. Other trouble building Python on platform X.
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. Is there a special lib for writing CGI scripts in Python?
4.30. Q. What other WWW tools are there for Python?
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. What happened to mktuple(), featuring in an example in the
Extensions manual?
5.8. Q. How do I call an object's method from C?
5.9. Q. How do I catch the output from print_error()?
5.10. Q. How do I access a module written in Python from 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?
6.4. Q. Why does Python use indentation for grouping of statements?
6.5. Q. Why are Python strings immutable?
6.6. Q. Why don't strings have methods like index() or sort(), like
lists?
6.7. Q. Why does Python use methods for some functionality
(e.g. list.index()) but functions for other (e.g. len(list))?
6.8. Q. Why can't I derive a class from built-in types (e.g. lists or
files)?
6.9. Q. Why must 'self' be declared and used explicitly in method
definitions and calls?
6.10. Q. Can't you emulate threads in the interpreter instead of
relying on an OS-specific thread implementation?
6.11. Q. Why can't lambda forms contain statements?
6.12. Q. Why is there no more efficient way of iterating over a dictionary
than first constructing the list of keys()?
7. Using Python on non-UNIX platforms
7.1. Q. Is there a Mac version of Python?
7.2. Q. Is there a DOS version of Python?
7.3. Q. Is there a Windows version of Python?
7.4. Q. Is there a Windows NT version of Python?
7.5. Q. Is there an OS/2 version of Python?
7.6. Q. Is there a VMS version of Python?
7.7. Q. What about IBM mainframes, or other esoteric non-UNIX
platforms?
7.8. Q. Where are the source or Makefiles for the non-UNIX versions?
7.9. Q. What is the status and support for the non-UNIX versions?
7.10. Q. I have the PC version but it appears to be only a binary.
Where's the library?
7.11. Q. Where's the documentation for the Mac or PC version?
7.12. 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 wizard, 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.cwi.nl/pub/python/python1.1.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.)
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.cwi.nl/pub/python/patch1.1.1>.
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.cwi.nl/pub/python/index.html>.
1.4. Q. How do I get documentation on Python?
A. The latest Python documentation set is always available by
anonymous ftp, e.g.
<URL:ftp://ftp.cwi.nl/pub/python/pythondoc-ps1.1.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.
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:
<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.funet.fi/pub/languages/python/>
<URL:ftp://ftp.sunet.se/pub/lang/python/>
<URL:ftp://unix.hensa.ac.uk/uunet/languages/python/>
<URL:ftp://ftp.sterlng.com/programming/languages/python/>
<URL:ftp://ftp.ibp.fr/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
<URL:mailto:python-list-request@cwi.nl>. 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.cwi.nl/pub/python/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".
1.7. Q. Is there a WWW page devoted to Python?
A. The official Python home page is
<URL:http://www.cwi.nl/~guido/Python.html>. Michael McLay at NIST
maintains a Python page at <URL:http://www.eeel.nist.gov/python/>.
1.8. Q. Is there a book on Python, or will there be one out soon?
A. I am writing one. Addison-Wesley is interested. Optimistically,
it will be ready for the press by mid-1995. Mark Lutz is working on
one too -- more details as they emerge.
1.9. 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.
1.10. Q. Are there short introductory papers or talks on Python?
A. A 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 is available by ftp as
<URL:ftp://ftp.cwi.nl/pub/python/nluug-paper.ps> and
<URL:ftp://ftp.cwi.nl/pub/python/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 1995 are
available as <URL:ftp://ftp.cwi.nl/pub/python/vhll-slides.ps>.
1.11. Q. How does the Python version numbering scheme work?
A. Python versions are numbered A.B.C. 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 release. 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!
1.12. Q. Are there other ftp sites that carry Python related material?
A. An interesting ftp site for Python users is ftp.markv.com; the
directory pub/python contains a growing collection of interesting
Python scripts <URL:ftp://ftp.markv.com/pub/python/>. To submit a
script for inclusion, place it together with a readme file (with
extension .readme) in the publicly writable directory
/incoming/python. This service is maintained by Lance Ellinghaus
<lance@markv.com>. (I've heard complaints about this service not
being very responsive -- try at your own risk.)
1.13. 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!
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.
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>.
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. 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 management application. Contact: <info@sunrise.com>.
Infoseek uses Python to implement their commercial WWW information
retrieval service <URL:http://www.infoseek.com/>. Contact:
<info@infoseek.com>.
Michael Powers of daVinci Time & Space is "writing tons-o-python for
interactive television entertainment." Contact: <powers@dvts.com>.
Paul Everitt of Connecting Minds is planning a Lotus Notes gateway.
Contact: <Paul.Everitt@cminds.com>. Or see their WWW server
<URL:http://www.cminds.com/>.
Individuals at many other companies are using Python for
internal development (witness their contributions to the Python
mailing list or newsgroup).
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>.
If you'd like to be included in the list above, send me email!
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 1.1.x) have been coming out roughly every 3 to
6 months for the past four years.
2.5. Q. When will the next version be released?
A. I am planning to release 1.2 in January 1995. It will contain
hooks into the implementation of the import command, a (still limited)
form of persistent objects, and the usual complement of bug fixes
(including many fixed memory leaks and thread problems).
2.6. Q. What new developments are expected for Python in the future?
A. There will be better ports to the Mac, DOS, Windows, Windows NT, and
OS/2. The Windows ports will support dynamically loaded modules using
DLLs.
Also planned is improved support for embedding Python in other
applications, e.g. by renaming most global symbols to have a "Py"
prefix and providing more documentation and threading support.
Some proposals were discussed at the recent Python workshop:
- persistent objects
- safe execution of untrusted Python code
- extensions to the import statement for managing packages
(groups of related modules)
- automatic generation of C/C++ interface glue
- interfaces to OMG IDL (== Interface Definition Language by
the Object Management Group)
- a portable GUI API (Graphical User Interface Application
Programmers Interface)
- module customization tools
- standardized documentation strings on module, class and
function objects
- the formation of a Python Steering Committee
- another Python Workshop
For more info, have a look at the WWW page for the last Python
Workshop <URL:http://www.eeel.nist.gov/python/workshop11-94/>.
2.7. 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.
3. Building Python
==================
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 building Python with STDWIN 0.9.8. on SGI IRIX.
A. Get STDWIN 0.9.9 <URL:ftp://ftp.cwi.nl/pub/stdwin/stdwin0.9.9.tar.gz>.
3.4. Q. Link errors building Python with STDWIN 0.9.9.
A. Probably routines like 'tereate', 'tenew' etc. The STDWIN 0.9.9
distribution requires that you add TWO libraries from stdwin to the
line for stdwin in the Setupfile. Use something like this (all on one
line!):
stdwin stdwinmodule.c -I$(STDWIN)/H $(STDWIN)/Packs/textedit/libtextedit.a $(STDWIN)/Ports/x11/libstdwin.a -lX11
3.5. Q. Link errors after rerunning the configure script.
A. It is generally necessary to run "make clean" after a configuration
change.
3.6. 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.
3.7. 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).
3.8. Q. Python built with gcc for the DEC Alpha doesn't work.
People have reported problems with both gcc 2.5.8 and 2.6.0. The DEC
OSF/1 cc compiler does not have these problems so it's probably gcc's
fault. One person reported that the problem went away when using -g
instead of -O so this may be an option if you insist on using gcc. If
someone tracks it down more completely I'd like to hear about it!
3.9. 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.
3.10. Q. Trouble building or linking with the GNU readline library.
A. Consider using readline 2.0. From the Python 1.1 README:
- 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 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.bugs <URL:news:gnu.bash.bugs> for
specific problems with the readline library (I don't get this group
here but I've been told that it is the place for readline bugs.)
3.11. Q. Trouble building Python on Linux.
A. (Adapted from Bennet Todd:)
It seems to work for some people but not for others.
This might depend on which Linux release you're using. Older Linux
releases had (understandably) fewer gracious hack to improve Unix
compatibility; really old Linux was Posix 1003.1 compatible, without
nearly as much Unix compatibility as one might have wished. Current
releases of Linux build most current free software, either BSD or
System V, with little or no trouble.
Besides that, there are quite a few different releases currently,
including MCC, Slackware, SLS, and Debian, and probably more. The
differences between their compilation environments are far smaller
than they used to be, but they're still updated with different
frequencies. It's likely that the current Slackware works fine, as it
currently seems to be the most popular. But that's just a guess.
3.12. Q. Trouble with prototypes on Ultrix.
A. Ultrix cc seems broken -- use gcc, or edit config.h to #undef
HAVE_PROTOTYPES.
3.13. Q. Trouble with posix.listdir on NeXTSTEP 3.2.
A. The problem seems to be that that the NeXT posix library and the
NeXT dynamic loading library are incompatible. Mike Carlton reports
that the following worked for him (from a clean 1.1 distribution):
1) ./configure
2) edited config.status and changed
OPT='-O'
to
OPT='-posix -O'
3) edited Python/import.c and commented out the section
#if defined(NeXT) || defined(WITH_RLD)
#define DYNAMIC_LINK
#define USE_RLD
#endif
this disables dynamic loading
4) make
3.14. Q. Other trouble building Python on platform X.
A. Please email me the details <URL:mailto:guido@cwi.nl> 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.
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)
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 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 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 wring 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.cwi.nl/pub/python/extensions.tar.gz> and follow the
instructions there. Note: the X related modules are still somewhat
flakey, so don't try this unless you alread know a bit or two about
building X applications on your platform.
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.
- There's a neat object-oriented interface to the Tcl/Tk widget set,
called Tkinter. As of python 1.1, it is part of the standard Python
distribution -- all you need to do is enable it in Modules/Setup
(provided you have already installed Tk and Tcl). 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 John Ousterhout's home page
<URL:http://playground.Sun.COM:80/~ouster/>.
- The standard Python distribution comes with an interface to STDWIN,
a platform-independent low-level windowing interface. You have to ftp
the source for STDWIN separately,
e.g. <URL:ftp://ftp.cwi.nl/pub/stdwin/> or gatekeeper.dec.com in
pub/misc/stdwin <URL:ftp://gatekeeper.dec.com/pub/misc/stdwin/>.
STDWIN runs under X11 or the Mac; a Windows port has been attempted
but I can't seem to get it working. Note that STDWIN is really not
powerful enough to implement a modern GUI (no widgets, etc.) and that
I don't have the time to maintain or extend it, so you may be better
off using Tkinter or the Motif interface, unless you require
portability to the Mac (which is also offered by SUIT, by the way --
see below).
- For SGI IRIX only, there's an interface 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/>).
- 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) in the Extensions set, which is separately ftp'able
<URL:ftp://ftp.cwi.nl/pub/python/extensions.tar.gz>.
- There's an interface to SUIT, the U of Virginia's Simple User
Interface Toolkit; it can be ftp'ed from
<URL:ftp://uvacs.cs.virginia.edu/pub/suit/python/SUIT_python.tar.Z>.
A PC binary of Python 1.0.2 compiled with DJGPP and with SUIT support
built-in has been made available by Antonio Costa
<URL:ftp://asterix.inescn.pt/pub/PC/python/pyt102su.exe> (a
self-extracting archive). Note that the UVa people themselves have
expressed doubts about SUIT, and are planning to build a Python GUI
API based upon Tk (though not necessarily on Tkinter); see
<URL:http://server.cs.virginia.edu/~tnb2d/IT/IT.html>.
- There's an interface to WAFE, a Tcl interface to the X11 Motif and
Athena widget sets. Last I heard about it it was included in the WAFE
1.0 prerelease
<URL:ftp://ftp.wu-wien.ac.at/pub/src/X11/wafe/wafe-1.0.tar.gz-prerelease>.
4.14. Q. Are there any interfaces to database packages in Python?
A. There's an interface to SYBASE by John Redford
<jredford@lehman.com>.
There's also an interface to metalbase by Lance Ellinghaus
<lance@markv.com>; it is part of the separate Extensions distribution
<URL:ftp://ftp.cwi.nl/pub/python/extensions.tar.gz>.
Anthony Baxter <anthony.baxter@aaii.oz.au> has written an interface to
mSQL (mini-SQL). <URL:ftp://ftp.cwi.nl/pub/python/PymSQL.tar.gz>.
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.
Steve Majewski (or was it Tim Peters?) 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, there are some obscure bugs if your __del__ method does
complicated things such as updating dictionaries or lists or
references globals. I hope to have fixed these in release 1.2.
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. 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).
However if all you want is to pass environment variables to the
commands run by os.system() or os.popen(), there's a simple solution:
prefix the command string with a couple of variable assignments and
export statements. I guess the following would be universal for popen
(untested):
import os
from commands import mkarg # nifty routine to add shell quoting
def epopen(cmd, mode, env = {}):
# env is a dictionary of environment variables
prefix = ''
for key, value in env.values():
prefix = prefix + '%s=%s\n' % (key, mkarg(value))
prefix = prefix + 'export %s\n' % key
return os.popen(prefix + cmd, mode)
4.19. Q. What is a class?
A. A class is the particular object type that is created by executing
a class statement.
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 word 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.
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.
Unfortunately, the current version is very platform-specific, because
each platform has its own compilation flags and libraries to link
with. You will probably have to edit the freeze.py file to point it
to the right directories and tell it about the compilation and link
flags for your platform. A new version will be released with Python
1.1 -- if you want a peek write to my colleague <Jack.Jansen@cwi.nl>.
4.29. Q. Is there a special lib for writing CGI scripts in Python?
A. There's documentation and code for a cgi.py module by Michael McLay
<mclay@eeel.nist.gov> available from
<URL:http://www.eeel.nist.gov/python/>
(For the curious: CGI or Common Gateway Interface is the protocol
between HTTP servers (WWW servers) and programs/scripts they run to
perform queries and other tasks that require returning a dynamically
generated document.)
4.30. Q. What other WWW tools are there for Python?
A. The standard library has a module urllib, which can retrieve most
commonly used URL types (file, ftp, http, gopher).
The Demo2/www directory (Demo2 has to be retrieved separately from the
Python ftp sites <URL:ftp://ftp.cwi.nl/pub/python/demo2.tar.gz>)
contains some (really old) code to parse HTML and to display it.
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>.)
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. What happened to mktuple(), featuring in an example in the
Extensions manual?
A. It's a typo, I meant newtupleobject() (see previous question).
5.8. 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)".
5.9. 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.
FWIW, there seems to be a small problem that requires the 'softspace'
attribute to be defined too (I can't remember exact details of the
problem).
* 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.
5.10. 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.
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.
A partial solution will appear in release 1.2. This will also provide
a partial solution to the problem of a 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 consensus (yet) on whether and how
to do range tests.
6.4. 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 there can be no conflicting
coding styles. 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!
6.5. 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)
6.6. 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).
6.7. 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.
6.8. 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.
6.9. 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.
6.10. 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.
6.11. 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 functions.
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!
6.12. 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.
7. Using Python on non-UNIX platforms
=====================================
7.1. Q. Is there a Mac version of Python?
A. Yes. It is on most ftp sites carrying Python as python.sea.hqx,
e.g. <URL:ftp://ftp.cwi.nl/pub/python/python_1.1.sea.hqx> -- this is a
self-extracting archive containing the application binary as well as
the Lib modules.
7.2. Q. Is there a DOS version of Python?
A. Yes. More than one, actually: 16python.exe runs in standard DOS
mode on 186 CPUs or higher; 32python.exe uses a DOS extender and only
runs on a 386 or higher CPUs. Although 16python.exe does not pass the
test set because test_grammar is too big for the parser, it actually
has about 270 kbyte of allocatable heap space, which is sufficient for
fairly large programs. 32python.exe is distributed as a tar file
containing the required DOS extended and 387 emulator. Both are on
most ftp sites carrying Python.
The file dosbuild.tar.gz on the standard ftp sites contains
rudimentary Makefiles and instructions
<URL:ftp://ftp.cwi.nl/pub/python/dosbuild.tar.gz>.
7.3. Q. Is there a Windows version of Python?
A. Yes. Use qwpython.exe. The only problem with it: ^C
unconditionally kills the entire program -- it does not raise
KeyboardInterrupt. You can also run 16python.exe or 32python.exe in a
"DOS box", but qwpython.exe appears to be slightly faster.
There aren't any Makefiles at this moment. Sorry.
Using Win32s (a free NT compatibility package by Microsoft) you can
also use the NT version by Mark Hammond -- the Win32s package is also
in that directory (you'll need several MB of disk space to install
it). See the next question.
7.4. Q. Is there a Windows NT version of Python?
A. Yes. Mark Hammond <MHammond@cmutual.com.au> has built a full NT
port. This supports using DLLs for dynamic loading of Python modules,
and 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.cwi.nl/pub/python/nt/> -- most mirrors
will also have this.
Sam Rushing <rushing@squirl.oau.org> once announced he knows how to
build Python for the Windows NT on the DEC Alpha AXP.
Note that currently there is no unified compilation environment for
all NT platforms -- hopefully Microsoft will fix this with the release
of Visual C++ 2.0.
7.5. Q. Is there an OS/2 version of Python?
A. Yes. You can ftp it from the usual places as pyth_os2.zip, e.g.
<URL:ftp://ftp.cwi.nl/pub/python/pyth_os2.zip>. This contains both an
executable and Makefiles for those fortunate enough to have a C
compiler.
7.6. Q. Is there a VMS version of Python?
A. I think not (yet). This question has been asked on the list
several times and I've never seen an answer. Maybe someone with a VMS
C compiler could attempt a port? Probably coming up with proper
Makefiles, config.h and config.c should be sufficient. If you succeed
(or even if you get stuck halfway), please let me know! (Info as of
23 September 1994.)
7.7. Q. What about IBM mainframes, or other esoteric non-UNIX
platforms?
A. Basically, the same story as for VMS... (Info as of 23 September
1994.)
7.8. Q. Where are the source or Makefiles for the non-UNIX versions?
A. The standard sources can (almost) be used. See the previous
questions for availability of Makefiles/projects or patches. If you
find things in the standard sources that don't work on your particular
platform, please let me know and I'll integrate a solution into the
next release of the standard sources. If you submit a fix, please use
some kind of #ifdef so as to keep the source working for other
platforms. In particular, if the patch works around the availability
of a particular function of header file, you should mimic the
HAVE_... style used by the configure script -- you can then submit a
config.h file for a particular platform so there are no absolutely
platform-specific #ifdefs in the rest of the sources.
7.9. 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>.
7.10. Q. I have the 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 pythonlib<version>.tar.gz from most ftp sites
carrying Python; this is a subset of the distribution containing just
those file, e.g.
<URL:ftp://ftp.cwi.nl/pub/python/pythonlib1.1.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
7.11. Q. Where's the documentation for the Mac or PC version?
A. There isn't any. The documentation for the Unix version also
applies to the Mac and PC versions. Where applicable, differences
are indicated in the text.
7.12. 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."