1994-01-26 06:20:16 -04:00
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Subject: FAQ: Python -- an object-oriented language
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1994-04-11 08:06:22 -03:00
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Newsgroups: comp.lang.python,comp.answers,news.answers
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Followup-to: comp.lang.python
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1994-01-26 06:20:16 -04:00
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From: guido@cwi.nl (Guido van Rossum)
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Reply-to: guido@cwi.nl (Guido van Rossum)
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Approved: news-answers-request@MIT.Edu
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Archive-name: python-faq/part1
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1994-06-13 12:13:56 -03:00
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Version: 1.9
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Last-modified: 8 June 1994
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1994-01-26 06:20:16 -04:00
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This article contains answers to Frequently Asked Questions about
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Python (an object-oriented interpreted programming language -- see
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the answer to question 1.1 for a short overview).
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Copyright 1993, 1994 Guido van Rossum. Unchanged electronic
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redistribution of this FAQ is allowed. Printed redistribution only
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with permission of the author. No warranties.
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Author's address:
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Guido van Rossum
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CWI, dept. CST
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Kruislaan 413
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P.O. Box 94079
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1090 GB Amsterdam
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The Netherlands
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Email: guido@cwi.nl
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The latest version of this FAQ is available by anonymous ftp from
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1994-06-13 12:13:56 -03:00
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ftp.cwi.nl [192.16.191.128] in the directory /pub/python, with
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1994-01-26 06:20:16 -04:00
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filename python-FAQ. It will also be posted regularly to the
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1994-04-11 08:06:22 -03:00
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newsgroups comp.answers and comp.lang.python.
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1994-01-26 06:20:16 -04:00
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Many FAQs, including this one, are available by anonymous ftp from
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rtfm.mit.edu [18.70.0.209] in the directory pub/usenet/news.answers.
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The name under which a FAQ is archived appears in the Archive-name line
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at the top of the article. This FAQ is archived as python-faq/part1.
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There's a mail server on that machine which will send you files from
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the archive by e-mail if you have no ftp access. You send a e-mail
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message to mail-server@rtfm.mit.edu containing the single word help in
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the message body to receive instructions.
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This FAQ is divided in the following chapters:
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1. General information and availability
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2. Python in the real world
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3. Building Python
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4. Programming in Python
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5. Extending Python
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6. Python's design
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7. Using Python on non-UNIX platforms
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To find the start of a particular chapter, search for the chapter number
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followed by a dot and a space at the beginning of a line (e.g. to
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find chapter 4 in vi, type /^4\. /).
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Here's an overview of the questions per chapter:
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1. General information and availability
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1.1. Q. What is Python?
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1.2. Q. Why is it called Python?
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1.3. Q. How do I obtain a copy of the Python source?
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1.4. Q. How do I get documentation on Python?
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1994-05-31 06:18:50 -03:00
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1.5. Q. Are there other ftp sites that mirror the Python distribution?
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1.6. Q. Is there a newsgroup or mailing list devoted to Python?
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1.7. Q. Is there a book on Python, or will there be one out soon?
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1.8. Q. Are there any published articles about Python that I can quote?
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1.9. Q. How does the Python version numbering scheme work?
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1.10. Q. Are there other ftp sites that carry Python related material?
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1994-06-13 12:13:56 -03:00
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1.11. Q. Are there copyright restrictions on the use of Python?
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1994-01-26 06:20:16 -04:00
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2. Python in the real world
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2.1. Q. How many people are using Python?
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2.2. Q. Have any significant projects been done in Python?
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2.3. Q. Are there any commercial projects going on using Python?
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1994-01-26 13:23:37 -04:00
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2.4. Q. How stable is Python?
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2.5. Q. What new developments are expected for Python in the future?
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1994-01-26 06:20:16 -04:00
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3. Building Python
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1994-02-15 11:52:27 -04:00
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3.1. Q. Is there a test set?
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3.2. Q. When running the test set, I get complaints about floating point
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1994-01-26 06:20:16 -04:00
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operations, but when playing with floating point operations I cannot
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find anything wrong with them.
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1994-02-15 11:52:27 -04:00
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3.3. Q. Link errors building Python with STDWIN on SGI IRIX.
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3.4. Q. Link errors after rerunning the configure script.
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3.5. Q. The python interpreter complains about options passed to a
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1994-02-02 10:12:45 -04:00
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script (after the script name).
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1994-02-15 11:52:27 -04:00
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3.6. Q. When building on the SGI, make tries to run python to create
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1994-04-11 08:06:22 -03:00
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glmodule.c, but python hasn't been built or installed yet.
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1994-05-04 10:28:51 -03:00
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3.7. Q. Other trouble building Python 1.0.2 on platform X.
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1994-01-26 06:20:16 -04:00
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4. Programming in Python
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1994-02-02 10:12:45 -04:00
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4.1. Q. Is there a source code level debugger with breakpoints, step,
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etc.?
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4.2. Q. Can I create an object class with some methods implemented in
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1994-01-26 06:20:16 -04:00
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C and others in Python (e.g. through inheritance)? (Also phrased as:
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Can I use a built-in type as base class?)
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1994-02-02 10:12:45 -04:00
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4.3. Q. Is there a curses/termcap package for Python?
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4.4. Q. Is there an equivalent to C's onexit() in Python?
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4.5. Q. When I define a function nested inside another function, the
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1994-01-26 06:20:16 -04:00
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nested function seemingly can't access the local variables of the
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outer function. What is going on? How do I pass local data to a
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nested function?
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1994-02-02 10:12:45 -04:00
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4.6. Q. How do I iterate over a sequence in reverse order?
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4.7. Q. My program is too slow. How do I speed it up?
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4.8. Q. When I have imported a module, then edit it, and import it
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1994-01-26 06:20:16 -04:00
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again (into the same Python process), the changes don't seem to take
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place. What is going on?
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1994-02-02 10:12:45 -04:00
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4.9. Q. I have a module in which I want to execute some extra code when it
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is run as a script. How do I find out whether I am running as a
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script?
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1994-06-13 12:13:56 -03:00
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4.10. Q. I try to run a program from the Demo directory but it fails with
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ImportError: No module named ...; what gives?
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4.11. Q. What GUI toolkits exist for Python?
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4.12. Q. Are there any interfaces to commercial database in Python?
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1994-01-26 06:20:16 -04:00
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5. Extending Python
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5.1. Q. Can I create my own functions in C?
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5.2. Q. Can I create my own functions in C++?
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1994-06-13 12:13:56 -03:00
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5.3. Q. How can I execute arbitrary Python statements from C?
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5.4. Q. How can I evaluate an arbitrary Python expression from C?
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5.5. Q. How do I extract C values from a Python object?
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5.6. Q. How do I use mkvalue() to create a tuple of arbitrary length?
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1994-01-26 06:20:16 -04:00
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6. Python's design
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6.1. Q. Why isn't there a generic copying operation for objects in
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Python?
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6.2. Q. Why isn't there a generic way to implement persistent objects
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in Python? (Persistent == automatically saved to and restored from
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disk.)
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6.3. Q. Why isn't there a switch or case statement in Python?
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1994-05-31 06:18:50 -03:00
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6.4. Q. Why does Python use indentation for grouping of statements?
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1994-01-26 06:20:16 -04:00
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7. Using Python on non-UNIX platforms
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1994-02-15 11:52:27 -04:00
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7.1. Q. Is there a Mac version of Python?
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7.2. Q. Is there a DOS version of Python?
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7.3. Q. Is there a Windows version of Python?
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7.4. Q. Is there a Windows NT version of Python?
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1994-06-13 12:13:56 -03:00
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7.5. Q. Is there an OS/2 version of Python?
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7.6. Q. I have the PC version but it appears to be only a binary.
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Where's the library?
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7.7. Q. Where's the documentation for the Mac or PC version?
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7.8. Q. The Mac (PC) version doesn't seem to have any facilities for
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1994-02-15 11:52:27 -04:00
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creating or editing programs apart from entering it interactively, and
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there seems to be no way to save code that was entered interactively.
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How do I create a Python program on the Mac (PC)?
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1994-01-26 06:20:16 -04:00
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To find a particular question, search for the question number followed
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by a dot, a space, and a Q at the beginning of a line (e.g. to find
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question 4.2 in vi, type /^4\.2\. Q/).
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1. General information and availability
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=======================================
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1.1. Q. What is Python?
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A. Python is an interpreted, interactive, object-oriented programming
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language. It incorporates modules, exceptions, dynamic typing, very
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high level dynamic data types, and classes. Python combines
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remarkable power with very clear syntax. It has interfaces to many
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system calls and libraries, as well as to various window systems, and
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is extensible in C or C++. It is also usable as an extension language
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for applications that need a programmable interface. Finally, Python
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is portable: it runs on many brands of UNIX, on the Mac, and on
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MS-DOS.
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To find out more, the best thing to do is to start reading the
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tutorial from the documentation set (see a few questions further
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down).
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1.2. Q. Why is it called Python?
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A. Apart from being a computer wizard, I'm also a fan of "Monty
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Python's Flying Circus" (a BBC comedy series from the seventies, in
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the -- unlikely -- case you didn't know). It occurred to me one day
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that I needed a name that was short, unique, and slightly mysterious.
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And I happened to be reading some scripts from the series at the
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time... So then I decided to call my language Python. But Python is
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not a joke. And don't you associate it with dangerous reptiles
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either!
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1994-01-26 06:20:16 -04:00
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1.3. Q. How do I obtain a copy of the Python source?
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A. The latest Python source distribution is always available by
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1994-06-13 12:13:56 -03:00
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anonymous ftp from ftp.cwi.nl [192.16.191.128] in the directory
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1994-05-04 10:28:51 -03:00
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/pub/python, with filename python<version>.tar.gz. (Old versions may
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have an extension of .Z, indicating use of "compress" compression.)
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It is a gzip'ed tar file containing the complete C source, LaTeX
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documentation, Python library modules, example programs, and several
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useful pieces of freely distributable software. This will compile and
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run out of the box on most UNIX platforms. At the time of writing,
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<version> is 1.0.2. (See section 7 for non-UNIX information.)
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1994-01-26 06:20:16 -04:00
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1.4. Q. How do I get documentation on Python?
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A. The latest Python documentation set is always available by
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1994-06-13 12:13:56 -03:00
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anonymous ftp from ftp.cwi.nl [192.16.191.128] in the directory
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1994-05-04 10:28:51 -03:00
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/pub/python, with filename pythondoc-ps<version>.tar.gz. It is a
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gzip'ed tar file containing PostScript files of the reference manual,
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the library manual, and the tutorial. At the time of writing
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<version> is 1.0.2. Note that the library manual is the most
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important one of the set, as much of Python's power stems from the
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standard or built-in types, functions and modules, all of which are
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described here. PostScript for a high-level description of Python is
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in the file nluug-paper.ps.
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1994-01-26 06:20:16 -04:00
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1994-05-31 06:18:50 -03:00
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1.5. Q. Are there other ftp sites that mirror the Python distribution?
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A. The following sites keep mirrors of the Python distribution:
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1994-01-26 06:20:16 -04:00
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Site IP address Directory
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1994-06-13 12:13:56 -03:00
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gatekeeper.dec.com 16.1.0.2 /pub/plan/python
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1994-01-26 06:20:16 -04:00
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ftp.uu.net 192.48.96.9 /languages/python
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ftp.wustl.edu 128.252.135.4 /graphics/graphics/sgi-stuff/python
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1994-05-31 06:18:50 -03:00
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ftp.funet.fi 128.214.6.100 /pub/languages/python
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1994-04-11 08:06:22 -03:00
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ftp.fu-berlin.de 130.133.4.50 /pub/unix/languages/python (*python* only)
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1994-05-31 06:18:50 -03:00
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ftp.sunet.se 130.238.127.3 /pub/lang/python
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1994-01-26 06:20:16 -04:00
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1994-04-11 08:06:22 -03:00
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Or try archie on e.g. python1.0 to locate the nearest copy of that
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version...
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1994-01-26 06:20:16 -04:00
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1994-05-31 06:18:50 -03:00
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1.6. Q. Is there a newsgroup or mailing list devoted to Python?
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1994-01-26 06:20:16 -04:00
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1994-04-11 08:06:22 -03:00
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A. There is a newsgroup, comp.lang.python, and a mailing list. The
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newsgroup and mailing list are gatewayed into each other -- if you can
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read news it is not necessary to subscribe to the mailing list. Send
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e-mail to python-list-request@cwi.nl to (un)subscribe to the mailing
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1994-06-13 12:13:56 -03:00
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list. Once you're on, send e-mail tp python-list@cwi.nl to send mail
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to the entire mailing list and newsgroup.
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1994-01-26 06:20:16 -04:00
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1994-05-31 06:18:50 -03:00
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1.7. Q. Is there a book on Python, or will there be one out soon?
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1994-01-26 06:20:16 -04:00
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1994-06-13 12:13:56 -03:00
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A. Unfortunately, not yet. Mark Lutz and I are planning to write one,
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but we are still a in very preliminary stage. If you would like to
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beat us at it and get rich from book royalties, go ahead!
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1994-01-26 06:20:16 -04:00
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1994-05-31 06:18:50 -03:00
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1.8. Q. Are there any published articles about Python that I can quote?
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1994-01-26 06:20:16 -04:00
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A. So far the only refereed and published article that describes
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Python in some detail is:
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Guido van Rossum and Jelke de Boer, "Interactively Testing Remote
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Servers Using the Python Programming Language", CWI Quarterly, Volume
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4, Issue 4 (December 1991), Amsterdam, pp 283-303.
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LaTeX source for this paper is available as part of the Python source
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distribution.
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A more recent high-level description of Python is:
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Guido van Rossum, "An Introduction to Python for UNIX/C
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Programmers", in the proceedings of the NLUUG najaarsconferentie
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1993 (dutch UNIX users group meeting november 1993).
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PostScript for this paper and for the slides used for the accompanying
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presentation can be found in the ftp directory mentioned a few
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questions earlier, with filenames nluug-paper.ps and nluug-slides.ps,
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respectively.
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1994-05-31 06:18:50 -03:00
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1.9. Q. How does the Python version numbering scheme work?
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1994-01-26 13:23:37 -04:00
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A. Python versions are numbered A.B.C. A is the major version number
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-- it is only incremented for major changes in functionality or source
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structure. B is the minor version number, incremented for less
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earth-shattering changes to a release. C is the patchlevel -- it is
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incremented for each new release. Note that in the past, patches have
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added significant changes; in fact the changeover from 0.9.9 to 1.0.0
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was the first time that either A or B changed!
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1994-05-31 06:18:50 -03:00
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1.10. Q. Are there other ftp sites that carry Python related material?
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1994-04-11 08:06:22 -03:00
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A. An interesting ftp site for Python users is ftp.markv.com
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(192.122.251.1); the directory pub/python contains a growing
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collection of interesting Python scripts. To submit a script for
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inclusion, place it together with a readme file (with extension
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.readme) in the publicly writable directory /incoming/python. This
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service is maintained by Lance Ellinghouse <lance@markv.com>.
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1994-06-13 12:13:56 -03:00
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|
1.11. 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, display those copyrights in any
|
|
|
|
documentation about Python that you produce, 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).
|
1994-04-11 08:06:22 -03:00
|
|
|
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
2. Python in the real world
|
|
|
|
===========================
|
|
|
|
|
|
|
|
2.1. Q. How many people are using Python?
|
|
|
|
|
1994-04-11 08:06:22 -03:00
|
|
|
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
|
1994-06-13 12:13:56 -03:00
|
|
|
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.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
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
|
1994-04-11 08:06:22 -03:00
|
|
|
5,000 line multimedia teleconferencing tool, as well as many many
|
|
|
|
smaller programs.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
The University of Virginia uses Python to control a virtual reality
|
|
|
|
engine. Contact: Matt Conway <conway@virginia.edu>.
|
|
|
|
|
|
|
|
See also the next question.
|
|
|
|
|
|
|
|
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
|
|
|
|
manangement application. Contact: <info@sunrise.com>.
|
|
|
|
|
|
|
|
Individuals at many other companies are using Python for
|
|
|
|
internal development (witness their contributions to the Python
|
1994-04-11 08:06:22 -03:00
|
|
|
mailing list or newsgroup).
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
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>.
|
|
|
|
|
1994-01-26 13:23:37 -04:00
|
|
|
2.4. Q. How stable is Python?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
1994-05-04 10:28:51 -03:00
|
|
|
A. Very stable. While the current version number (1.0.2) would
|
1994-01-26 06:20:16 -04:00
|
|
|
suggest it is in the early stages of development, in fact new, stable
|
1994-01-26 13:23:37 -04:00
|
|
|
releases (numbered 0.9.x) have been coming out roughly every 3 to 6
|
|
|
|
months for the past four years.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
1994-01-26 13:23:37 -04:00
|
|
|
2.5. Q. What new developments are expected for Python in the future?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
1994-06-13 12:13:56 -03:00
|
|
|
A. A proposal is being worked out to change the semantics of operator
|
|
|
|
overloading (__add__, __mul__ etc.) to make them more useful for
|
|
|
|
implementing types that don't resemble numbers. Additions will be
|
|
|
|
__call__ (to call an instance as if it were a function) and __eq__,
|
|
|
|
_lt__ etc. (to override individual comparisons). A (new) pthreads
|
|
|
|
interface has been submitted which will be integrated in the next
|
|
|
|
release. The X11/Motif interface will be improved. There are ideas
|
|
|
|
about built-in help using strings placed into function objects, and
|
|
|
|
possibly a switch statement.
|
|
|
|
|
|
|
|
There will be better ports to the Mac, DOS, Windows, Windows NT, and
|
|
|
|
OS/2. The Windows ports will support dynalically loaded modules using
|
|
|
|
DLLs. The Mac port may also support dynamic loading of modules.
|
|
|
|
There may be a Windows version of STDWIN.
|
|
|
|
|
|
|
|
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.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
|
|
|
|
3. Building Python
|
|
|
|
==================
|
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
3.1. Q. Is there a test set?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
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.
|
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
3.2. Q. When running the test set, I get complaints about floating point
|
1994-01-26 06:20:16 -04:00
|
|
|
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.
|
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
3.3. Q. Link errors building Python with STDWIN on SGI IRIX.
|
1994-02-02 10:12:45 -04:00
|
|
|
|
|
|
|
A. Rebuild STDWIN, specifying "CC=cc -cckr" in the Makefile.
|
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
3.4. Q. Link errors after rerunning the configure script.
|
1994-02-02 10:12:45 -04:00
|
|
|
|
|
|
|
A. It is generally necessary to run "make clean" after a configuration
|
|
|
|
change.
|
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
3.5. Q. The python interpreter complains about options passed to a
|
1994-02-02 10:12:45 -04:00
|
|
|
script (after the script name).
|
|
|
|
|
|
|
|
A. You are probably linking with GNU getopt, e.g. through -liberty.
|
|
|
|
Don't. (If you are using this because you link with -lreadline, use
|
|
|
|
the readline distributed with Python instead.)
|
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
3.6. Q. When building on the SGI, make tries to run python to create
|
1994-02-02 10:12:45 -04:00
|
|
|
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).
|
|
|
|
|
1994-05-04 10:28:51 -03:00
|
|
|
3.7. Q. Other trouble building Python 1.0.2 on platform X.
|
1994-01-26 13:23:37 -04:00
|
|
|
|
|
|
|
A. Please email the details to <guido@cwi.nl> and I'll look into it.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
|
|
|
|
4. Programming in Python
|
|
|
|
========================
|
|
|
|
|
1994-02-02 10:12:45 -04:00
|
|
|
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
|
1994-01-26 06:20:16 -04:00
|
|
|
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)
|
|
|
|
|
1994-02-02 10:12:45 -04:00
|
|
|
4.3. Q. Is there a curses/termcap package for Python?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
A. No, but you can use the "alfa" (== character cell) version of
|
|
|
|
STDWIN. (STDWIN == Standard Windows, a portable windowing system
|
1994-02-15 11:52:27 -04:00
|
|
|
interface by the same author, URL: ftp://ftp.cwi.nl/pub/stdwin.)
|
1994-01-26 06:20:16 -04:00
|
|
|
This will also prepare your program for porting to windowing
|
|
|
|
environments such as X11 or the Macintosh.
|
|
|
|
|
1994-02-02 10:12:45 -04:00
|
|
|
4.4. Q. Is there an equivalent to C's onexit() in Python?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
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.
|
|
|
|
|
1994-02-02 10:12:45 -04:00
|
|
|
4.5. Q. When I define a function nested inside another function, the
|
1994-01-26 06:20:16 -04:00
|
|
|
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
|
|
|
|
|
1994-02-02 10:12:45 -04:00
|
|
|
4.6. Q. How do I iterate over a sequence in reverse order?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
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 the the method
|
|
|
|
call overhead...
|
|
|
|
|
1994-02-02 10:12:45 -04:00
|
|
|
4.7. Q. My program is too slow. How do I speed it up?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
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).
|
|
|
|
|
1994-02-02 10:12:45 -04:00
|
|
|
4.8. Q. When I have imported a module, then edit it, and import it
|
1994-01-26 06:20:16 -04:00
|
|
|
again (into the same Python process), the changes don't seem to take
|
|
|
|
place. What is going on?
|
|
|
|
|
|
|
|
A. For efficiency reasons, 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 objects imported
|
|
|
|
thus.
|
|
|
|
|
1994-02-02 10:12:45 -04:00
|
|
|
4.9. 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. A module can find out its own module name by alooking at the
|
|
|
|
(predefined) global variable __name__. If this has the value
|
|
|
|
'__main__' you are running as a script. 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()
|
|
|
|
|
1994-06-13 12:13:56 -03:00
|
|
|
4.10. 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 which hasn't been configured on
|
|
|
|
your system. This especially happens with modules like "stdwin",
|
|
|
|
"gl", "Xt" or "Xm". For 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). 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 file "extensions.tar.gz" file from a Python ftp repository
|
|
|
|
(e.g. ftp.cwi.nl) and follow the instructions there. Note: the X
|
|
|
|
related modules are still somewhat flakey, so don't try this unless
|
|
|
|
you know a bit or two about building X applications on your platform.
|
|
|
|
|
|
|
|
4.11. Q. What GUI toolkits exist for Python?
|
|
|
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|
|
A. Depending on what platform(s) you are aiming at, there are several.
|
|
|
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|
|
|
- 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. from ftp.cwi.nl in pub/stdwin
|
|
|
|
or gatekeeper.dec.com in pub/misc/stdwin). STDWIN runs under X11 or
|
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|
|
the Mac; a Windows port is about to be released.
|
|
|
|
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|
|
- For SGI only, there's an interface to the complete GL (Graphics
|
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|
|
Library -- low level but very good 3D capabilities) as well as to
|
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|
|
FORMS (a buttons-and-sliders-etc package built on top of GL by Mark
|
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|
|
Overmars -- ftp'able from ftp.cs.ruu.nl in pub/SGI/FORMS).
|
|
|
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|
- There's an interface to X11, including the Athena and Motif widget
|
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|
|
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
|
|
|
|
from ftp.cwi.nl as pub/python/extensions.tar.gz.
|
|
|
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|
- There's an interface to SUIT, the U of Virginia's Simple User
|
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|
|
Interface Toolkit; it can be ftp'ed from uvacs.cs.virginia.edu as
|
|
|
|
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 on ftp site asterix.inescn.pt, directory
|
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|
|
pub/PC/python, file pyt102su.exe (a self-extracting archive).
|
|
|
|
|
|
|
|
- There's a neat interface to the Tcl/Tk widget set; ftp it from
|
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|
|
ftp.cwi.nl as pub/python/tkinter.tar.gz.
|
|
|
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|
|
- There's an interface to WAFE, a Tcl interface to the X11 Motif and
|
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|
|
Athena widget sets. Last I heard about it it was included in the
|
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|
|
WAFE 1.0 prerelease, ftp'able from ftp.wu-wien.ac.at as
|
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|
|
pub/src/X11/wafe/wafe-1.0.tar.gz-prerelease.
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|
4.12. Q. Are there any interfaces to commercial database in Python?
|
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|
|
A. There's an interface to SYBASE by John Redford
|
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|
|
<jredford@lehman.com>.
|
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|
There's also an interface to metalbase by Lance Ellinghouse
|
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|
|
<lance@markv.com>.
|
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|
1994-01-26 06:20:16 -04:00
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|
|
5. Extending Python
|
|
|
|
===================
|
|
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|
|
5.1. Q. Can I create my own functions in C?
|
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|
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|
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|
|
A. Yes, you can create built-in modules containing functions,
|
1994-02-02 10:12:45 -04:00
|
|
|
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.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
5.2. Q. Can I create my own functions in C++?
|
|
|
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|
|
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.
|
|
|
|
|
1994-06-13 12:13:56 -03:00
|
|
|
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.
|
|
|
|
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|
|
|
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.
|
|
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|
|
|
|
|
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).
|
|
|
|
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
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 concensus (yet) on whether and how
|
|
|
|
to do range tests.
|
|
|
|
|
1994-05-31 06:18:50 -03:00
|
|
|
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!
|
|
|
|
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
7. Using Python on non-UNIX platforms
|
|
|
|
=====================================
|
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
7.1. Q. Is there a Mac version of Python?
|
|
|
|
|
|
|
|
A. Yes. It is on most ftp sites carrying Python as python.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.
|
|
|
|
|
|
|
|
7.3. Q. Is there a Windows version of Python?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
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.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
7.4. Q. Is there a Windows NT version of Python?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
1994-02-15 11:52:27 -04:00
|
|
|
A. Yes. Use ntpython.exe. This is for Intel CPUs. If you want a
|
|
|
|
Windows user interface, use qwpython.exe.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
1994-06-13 12:13:56 -03:00
|
|
|
7.5. Q. Is there an OS/2 version of Python?
|
|
|
|
|
|
|
|
A. Yes. You can ftp it (from ftp.cwi.nl in pub/python, or from the
|
|
|
|
mirror sites) as pyth_os2.zip. This contains both an executable and
|
|
|
|
Makefiles for those fortunate enough to have a C compiler.
|
|
|
|
|
|
|
|
7.6. Q. I have the PC version but it appears to be only a binary.
|
|
|
|
Where's the library?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
A. You still need to copy the files from the distribution directory
|
1994-02-15 11:52:27 -04:00
|
|
|
"python/Lib" to your system. If you don't have the full distribution,
|
1994-05-04 10:28:51 -03:00
|
|
|
you can get the file pythonlib1.0.2.tar.gz from most ftp sites carrying
|
1994-02-15 11:52:27 -04:00
|
|
|
Python; this is a subset of the distribution containing just those
|
|
|
|
file.
|
1994-01-26 06:20:16 -04:00
|
|
|
|
1994-04-11 08:06:22 -03:00
|
|
|
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
|
|
|
|
|
1994-06-13 12:13:56 -03:00
|
|
|
7.7. Q. Where's the documentation for the Mac or PC version?
|
1994-01-26 06:20:16 -04:00
|
|
|
|
|
|
|
A. There isn't any. The documentation for the Unix version also
|
1994-02-15 11:52:27 -04:00
|
|
|
applies to the Mac and PC versions. Where applicable, differences
|
1994-01-26 06:20:16 -04:00
|
|
|
are indicated in the text.
|
|
|
|
|
1994-06-13 12:13:56 -03:00
|
|
|
7.8. Q. The Mac (PC) version doesn't seem to have any facilities for
|
1994-02-15 11:52:27 -04:00
|
|
|
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, I am quite happy with the Desk
|
|
|
|
Accessory called Sigma Edit; this doesn't require Multifinder or
|
|
|
|
System 7. I work like this: start the interpreter; edit a module file
|
|
|
|
using Sigma Edit; import and test it in the interpreter; edit again in
|
|
|
|
Sigma Edit; then use the built-in function reload() to re-read the
|
|
|
|
imported module; etc.
|
1994-04-11 08:06:22 -03:00
|
|
|
|
|
|
|
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."
|