Subject: FAQ: Python -- an object-oriented language

Newsgroups: comp.lang.python,comp.answers,news.answers

Followup-to: comp.lang.python

From: [email protected] (Guido van Rossum)

Reply-to: [email protected] (Guido van Rossum)

Expires: Sat, 1 Apr 1995 00:00:00 GMT

Supersedes: <[email protected]>

Approved: [email protected]

Archive-name: python-faq/part1

Submitted-by: Guido van Rossum <[email protected]>

Version: 1.20++

Last-modified: 16 Februari 1995

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: <[email protected]>

The latest version of this FAQ is available by anonymous ftp from

<URL:ftp://ftp.cwi.nl/pub/python/python-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>.

Many FAQs, including this one, are available by anonymous ftp

<URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/>. The name under

which a FAQ is archived appears in the Archive-name line at the top of

the article. This FAQ is archived as python-faq/part1

<URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/python-faq/part1>.

There's a mail server on that machine which will send you files from

the archive by e-mail if you have no ftp access. You send a e-mail

message to <[email protected]> containing the single word help

in the message body to receive instructions.

This FAQ is divided in the following chapters:

1. General information and availability

2. Python in the real world

3. Building Python

4. Programming in Python

5. Extending Python

6. Python's design

7. Using Python on non-UNIX platforms

To find the start of a particular chapter, search for the chapter number

followed by a dot and a space at the beginning of a line (e.g. to

find chapter 4 in vi, type /^4\. /).

Here's an overview of the questions per chapter:

1. General information and availability

1.1. Q. What is Python?

1.2. Q. Why is it called Python?

1.3. Q. How do I obtain a copy of the Python source?

1.4. Q. How do I get documentation on Python?

1.5. Q. 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 the Python documentation available on the WWW?

1.9. Q. Is there a book on Python, or will there be one out soon?

1.10. Q. Are there any published articles about Python that I can quote?

1.11. Q. Are there short introductory papers or talks on Python?

1.12. Q. How does the Python version numbering scheme work?

1.13. Q. How do I get a beta test version of Python?

1.14. Q. Are there other ftp sites that carry Python related material?

1.15. 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.

3.15. Q. How to configure dynamic loading on Lixux.

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?

4.31. Q. How do I run a subprocess with pipes connected to both input

and output?

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(), featured 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 scientist, 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 LaTeX source for the documentation is part of the source

distribution. If you don't have LaTeX, 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 (a separate file on the ftp

site).

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 <[email protected]> to

(un)subscribe to the mailing list. 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. Yes, <URL:http://www.cwi.nl/~guido/Python.html> is the official

Python home page. Michael McLay at NIST maintains a Python page at

<URL:http://www.eeel.nist.gov/python/>. And finally the emerging

Python Software Association has a preliminary web server at

<URL:http://www.cminds.com/python/> (eventually to be moved to

somewhere under python.org).

1.8. Q. Is the Python documentation available on the WWW?

A. Yes, see <URL:http://www.cwi.nl/~guido/Python.html> (Python's home

page). It contains pointers to hypertext versions of the whole

documentation set (as hypertext, not just PostScript).

If you wish to browse this collection of HTML files on your own

machine, it is available bundled up by anonymous ftp,

e.g. <URL:ftp://ftp.cwi.nl/pub/python/python-html.tar.gz>. (This

requires some manual patch-up for the references to images,

unfortunately.)

An Emacs-INFO set containing the library manual is also available by

ftp, e.g. <URL:ftp://ftp.cwi.nl/pub/python/python-lin-info.tar.gz>.

1.9. Q. Is there a book on Python, or will there be one out soon?

A. I am writing one. Publishers are interested. Optimistically, it

will be ready for the press by mid-1995. Mark Lutz is working on one

too -- more details as they emerge.

1.10. 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.

1.11. 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>.

1.12. 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!

Beta versions have an additional suffix of "-beta-N" for some small

number N. Note that (for instance) all versions labeled 1.2-beta-N

*precede* the actual release of 1.2.

1.13. Q. How do I get a beta test version of Python?

A. Write me. I might like you :-) Seriously, anybody who has

previously used and installed Python can become a beta tester, but I

expect feedback, so you have to write me first before I divulge the

secret location of the latest beta release.

1.14. 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

<[email protected]>. (I've heard complaints about this service not

being very responsive -- try at your own risk.)

1.15. 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 <[email protected]>.

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 use of 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: <[email protected]>.

Infoseek uses Python to implement their commercial WWW information

retrieval service <URL:http://www.infoseek.com/>. Contact:

<[email protected]>.

Michael Powers of daVinci Time & Space is "writing tons-o-python for

interactive television entertainment." Contact: <[email protected]>.

Paul Everitt of Connecting Minds is planning a Lotus Notes gateway.

Contact: <[email protected]>. 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 <[email protected]>.

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 February or early March 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. The linker probably complains that it can't find routines like

'tecreate', '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 gcc 2.5.8 up to 2.6.3. The DEC

OSF/1 cc compiler does not have these problems so it's likely a gcc

bug. The latest news is that (at least in the 1.2 beta release) the

problem still exists but only when you use -O, and only affects the

code generated from Python/marshal.c. A fix is to change "long i, n;"

into "int i, n;" in w_object().

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 and 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. In 1.1 and 1.1.1, there's a bug in the reference counting logic of

ternary pow() which is only tripped by very picky mallocs, like the

GNU malloc on Linux. This has been fixed in 1.2. To continue the

tests, just disable the tests of pow() with three arguments from

Lib/test/test_b2.py.

Apart from this, Python builds and runs fine on most Linux versions.

Bennet Todd has this to say on the subject:

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. You need to pass "-posix" to the C compiler, especially in the link

phase -- it uses the correct version of the C library. The easiest

way is perhaps to do "make OPT=-posix". You also need to remove this

NeXT-specific section from import.c:

#if defined(NeXT) || defined(WITH_RLD)

#define DYNAMIC_LINK

#define USE_RLD

#endif

3.14. Q. Other trouble building Python on platform X.

A. Please email the details to <[email protected]> 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.

3.15. Q. How to configure dynamic loading on Lixux.

A. (Thanks to Andrew Kuchling):

Linux requires that you use the GNU DLD library. The stages of using

dynamic libraries on Linux are:

1) Get dld 3.2.5 from a Linux site. Be careful here; the most recent

GNU version is 3.2.3, and doesn't support Linux; be sure to get it

from a Linux mirror, not a GNU mirror (3.2.4 should also work).

Compile it and install the library libdld.a somewhere; I used

/usr/local/lib.

Suitable URLs for the dld distribution are currently:

<URL:ftp://sunsite.unc.edu/pub/Linux/libs/dld-3.2.5.src.tar.gz> and

<URL:ftp://tsx-11.mit.edu/pub/linux/sources/libs/dld-3.2.5.src.tar.gz>.

There's also a binary distribution of it:

<URL:ftp://sunsite.unc.edu/pub/Linux/libs/dld-3.2.5.bin.tar.gz>.

2) Get Jack Jansen's DL library; its location is given in the

_Extending Python_ manual as ftp://ftp.cwi.nl/pub/dynload/. Compile

it and install libdl.a wherever you put libdld.a.

3) Run Python's configure script, giving it the --with-dl-dld option,

which requires a parameter giving the directory where you put the

libraries.

4) Recompile Python.

Note that once ELF is common in the Linux world, it will be much

easier (probably within a year) -- the standard SVR4-style dlopen()

interface is then available. This has already proved to work

perfectly using a beta version of the ELF-capable GCC for Linux.

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 rereading of a changed module, 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 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

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