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: Fri, 1 Sep 1995 00:00:00 GMT

Supersedes: <[email protected]>

Approved: [email protected]

Archive-name: python-faq/part1

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

Version: 1.25

Last-modified: 25 July 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.python.org/pub/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?

1.16. Q. Why was Python created in the first place?

1.17. Q. What happened to Tim Peters (the author of python-mode.el, and many

entertaining and enlightening pieces of Python email)?

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 after rerunning the configure script.

3.4. Q. The python interpreter complains about options passed to a

script (after the script name).

3.5. Q. When building on the SGI, make tries to run python to create

glmodule.c, but python hasn't been built or installed yet.

3.6. Q. Python built with gcc for the DEC Alpha doesn't work.

3.7. Q. I use VPATH but some targets are built in the source directory.

3.8. Q. Trouble building or linking with the GNU readline library.

3.9. Q. Trouble building Python on Linux.

3.10. Q. Trouble with prototypes on Ultrix.

3.11. Q. Trouble with posix.listdir on NeXTSTEP 3.2.

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

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

3.14. Q. Under Solaris 2.x, using GCC, how do I use shared libraries?

3.15. Q. Errors when linking with a shared library containing C++ code.

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. What WWW tools are there for Python?

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

and output?

4.31. Q. How do I call a function if I have the arguments in a tuple?

4.32. Q. How do I enable font-lock-mode for Python in Emacs?

4.33. Q. Is there an inverse to the format operator (a la C's scanf())?

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 3.1(1) version of Python?

7.4. Q. Is there a Windows NT version of Python?

7.5. Q. Is there a Windows 95 version of Python?

7.6. Q. Is there an OS/2 version of Python?

7.7. Q. Is there a VMS version of Python?

7.8. Q. What about IBM mainframes, or other esoteric non-UNIX

platforms?

7.9. Q. Where are the source or Makefiles for the non-UNIX versions?

7.10. Q. What is the status and support for the non-UNIX versions?

7.11. Q. I have a PC version but it appears to be only a binary.

Where's the library?

7.12. Q. Where's the documentation for the Mac or PC version?

7.13. 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.python.org/pub/python/src/python1.2.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.python.org/pub/python/src/patch1.1.1>. (At the time

of writing, no patches exist for 1.2.)

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.python.org/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.python.org/pub/python/doc/postscript.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://ftp.cwi.nl/pub/python/>

<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/mirrors/uunet/languages/python/>

<URL:ftp://ftp.sterling.com/programming/languages/python/>

<URL:ftp://ftp.ibp.fr/pub/python/>

<URL:ftp://ftp.switch.ch/software/sources/python/>

<URL:ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/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.python.org/pub/python/mail/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.python.org/> is the official Python home page.

At the time of writing, this page is not yet completely operational;

you may have a look at the old Python home page:

<URL:http://www.cwi.nl/~guido/Python.html> or at the U.S. copy:

<URL:http://www.python.org/~guido/Python.html>.

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

A. Yes, see <URL:http://www.python.org/> (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.python.org/pub/python/doc/html.tar.gz>.

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

ftp, e.g. <URL:ftp://ftp.python.org/pub/python/doc/lib-info.tar.gz>.

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

A. Mark Lutz is writing a Python book for O'Reilly and Associates, to

be published early 1996. See the outline (in PostScript):

<URL:http://www.python.org/workshops/1995-05/outlinep.eps>.

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.python.org/pub/python/doc/nluug-paper.ps> and

<URL:ftp://ftp.python.org/pub/python/doc/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.python.org/pub/python/doc/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!

1.16. Q. Why was Python created in the first place?

A. Here's a *very* brief summary of what got me started:

- I had extensive experience with implementing an interpreted language

in the ABC group at CWI, and from working with this group I had

learned a lot about language design. This is the origin of many

Python features, including the use of indentation for statement

grouping and the inclusion of very-high-level data types (although the

details are all different in Python).

- I had a number of gripes about the ABC language, but also liked many

of its features. It was impossible to extend the ABC language (or its

implementation) to remedy my complaints -- in fact its lack of

extensibility was one of its biggest problems.

- I had some experience with using Modula-2+ and talked with the

designers of Modula-3 (and read the M3 report). M3 is the origin of

the syntax and semantics used for exceptions, and some other Python

features.

- I was working in the Amoeba distributed operating system group at

CWI. We needed a better way to do system administration than by

writing either C programs or Bourne shell scripts, since Amoeba had

its own system call interface which wasn't easily accessible from the

Bourne shell. My experience with error handling

- It occurred to me that a scripting language with a syntax like ABC

but with access to the Amoeba system calls would fill the need. I

realized that it would be foolish to write an Amoeba-specific

language, so I decided that I needed a language that was generally

extensible.

- During the 1989 Christmas holidays, I had a lot of time on my hand,

so I decided to give it a try. During the next year, while still

mostly working on it in my own time, Python was used in the Amoeba

project with increasing success, and the feedback from colleagues made

me add many early improvements.

- In February 1991, after just over a year of development, I decided

to post to USENET. The rest is in the Misc/HISTORY file.

1.17. Q. What happened to Tim Peters (the author of python-mode.el, and many

entertaining and enlightening pieces of Python email)?

A. He worked at KSR (Kendall Square Research, a start-up building a

new kind of massively parallel processor). When KSR folded down a

couple of years ago, Tim lost his email access. He hasn't surfaced

on the net since then.

Missing-him-too-ly yours...

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>.

The University of California, Irvine uses a student administration

system called TELE-Vision written entirely in Python. Contact: Ray

Price <[email protected]>.

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.

Sunrise Software, 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/>.

KaPRE in Boulder, CO is using Python for on-site customization of C++

applications, rapid-prototyping/development,

language-based-components, and possibly more. This is pretty solid:

Python's being shipped with their tool-set now, to beta sites.

Contact: <[email protected]> (Mark Lutz).

Individuals at many other companies are using Python for internal

development or for as yet unannounced products (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. See my Work-In-Progress web page, currently at

<URL:http://www.python.org:~guido/WIP.html>, and the pages for the

Second Python Workshop (best reached via the Python home page,

<URL:http://www.python.org/>).

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 after rerunning the configure script.

A. It is generally necessary to run "make clean" after a configuration

change.

3.4. 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.

3.5. 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).

3.6. 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 this has been fixed in Python 1.2 by a

source change (I gave up waiting for a fixed gcc).

3.7. 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.

3.8. Q. Trouble building or linking with the GNU readline library.

A. Consider using readline 2.0. Some hints:

- 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.bug <URL:news:gnu.bash.bug> for

specific problems with the readline library (I don't read this group

but I've been told that it is the place for readline bugs).

3.9. Q. Trouble building Python on Linux.

A. If you're building Python 1.2, Slackware 2.2 has a buggy bash

(version 1.14.3) which breaks a sed script that is used to build

Modules/Makefile. Replace /bin/sh with /bin/ash in both makesetup and

Makefile.pre.in.

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 in 1.1(.1), 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

(if you run into trouble on an old Linux version, consider upgrading).

3.10. Q. Trouble with prototypes on Ultrix.

A. Ultrix cc seems broken -- use gcc, or edit config.h to #undef

HAVE_PROTOTYPES.

3.11. Q. Trouble with posix.listdir on NeXTSTEP 3.2.

A. (This often manifests itself as a weird error from the

compileall.py script run by "make libinstall".)

Don't use gcc, use the Next C compiler (cc). Even though it is

derived from (an old version of) gcc, its interpretation of the

"-posix" switch is different; in this particular case, cc is right and

gcc is wrong.

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

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.

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

A. There are two answers, depending on whether you are using the new

ELF object format or not.

For ELF, this seems to be the procedure (thanks to Martin von Loewis):

Compile Python to an ELF binary.

In addition, you have to use the following flags:

- when linking python: -rdynamic -ldl

- when compiling an object that goes into a shared module: -fPIC

- when linking a shared module: -shared -ldl

Furthermore, it appears that some Python releases did not understand

that Linux has dynamic linking. Python 1.2 did it right, but you

should check wether the generated config.h indicates the use of -ldl

(i.e. dlopen, dlsym). Finally, you can load a shared module by saying

'use foo'. Make sure the module is in your PYTHONPATH.

For pre-ELF systems (thanks to Andrew Kuchling):

Pre-ELF 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 <URL: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.

3.14. Q. Under Solaris 2.x, using GCC, how do I use shared libraries?

A. Use the linker in /usr/ucb/ld, not the GNU linker. The latter

cannot create shared libraries.

3.15. Q. Errors when linking with a shared library containing C++ code.

A. Link the main Python binary with C++. Change the definition of

LINKCC in Modules/Makefile to be your C++ compiler. You may have to

edit config.c slightly to make it compilable with C++.

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