Subject: FAQ: Python -- an object-oriented language Newsgroups: comp.lang.python,comp.answers,news.answers Followup-to: comp.lang.python From: guido@cwi.nl (Guido van Rossum) Reply-to: guido@cwi.nl (Guido van Rossum) Expires: Mon, 1 May 1995 00:00:00 GMT Supersedes: Approved: news-answers-request@MIT.Edu Archive-name: python-faq/part1 Submitted-by: Guido van Rossum Version: 1.23 Last-modified: 10 April 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: The latest version of this FAQ is available by anonymous ftp from . It will also be posted regularly to the newsgroups comp.answers and comp.lang.python . Many FAQs, including this one, are available by anonymous ftp . 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 . 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 containing the single word help in the message body to receive instructions. This FAQ is divided in the following chapters: 1. General information and availability 2. Python in the real world 3. Building Python 4. Programming in Python 5. Extending Python 6. Python's design 7. Using Python on non-UNIX platforms To find the start of a particular chapter, search for the chapter number followed by a dot and a space at the beginning of a line (e.g. to find chapter 4 in vi, type /^4\. /). Here's an overview of the questions per chapter: 1. General information and availability 1.1. Q. What is Python? 1.2. Q. Why is it called Python? 1.3. Q. How do I obtain a copy of the Python source? 1.4. Q. How do I get documentation on Python? 1.5. Q. Are there other ftp sites that mirror the Python distribution? 1.6. Q. Is there a newsgroup or mailing list devoted to Python? 1.7. Q. Is there a WWW page devoted to Python? 1.8. Q. Is the Python documentation available on the WWW? 1.9. Q. Is there a book on Python, or will there be one out soon? 1.10. Q. Are there any published articles about Python that I can quote? 1.11. Q. Are there short introductory papers or talks on Python? 1.12. Q. How does the Python version numbering scheme work? 1.13. Q. How do I get a beta test version of Python? 1.14. Q. Are there other ftp sites that carry Python related material? 1.15. Q. Are there copyright restrictions on the use of Python? 2. Python in the real world 2.1. Q. How many people are using Python? 2.2. Q. Have any significant projects been done in Python? 2.3. Q. Are there any commercial projects going on using Python? 2.4. Q. How stable is Python? 2.5. Q. When will the next version be released? 2.6. Q. What new developments are expected for Python in the future? 2.7. Q. Is it reasonable to propose incompatible changes to Python? 3. Building Python 3.1. Q. Is there a test set? 3.2. Q. When running the test set, I get complaints about floating point operations, but when playing with floating point operations I cannot find anything wrong with them. 3.3. Q. Link errors building Python with STDWIN 0.9.8. on SGI IRIX. 3.4. Q. Link errors building Python with STDWIN 0.9.9. 3.5. Q. Link errors after rerunning the configure script. 3.6. Q. The python interpreter complains about options passed to a script (after the script name). 3.7. Q. When building on the SGI, make tries to run python to create glmodule.c, but python hasn't been built or installed yet. 3.8. Q. Python built with gcc for the DEC Alpha doesn't work. 3.9. Q. I use VPATH but some targets are built in the source directory. 3.10. Q. Trouble building or linking with the GNU readline library. 3.11. Q. Trouble building Python on Linux. 3.12. Q. Trouble with prototypes on Ultrix. 3.13. Q. Trouble with posix.listdir on NeXTSTEP 3.2. 3.14. Q. Other trouble building Python on platform X. 3.15. Q. How to configure dynamic loading on Lixux. 4. Programming in Python 4.1. Q. Is there a source code level debugger with breakpoints, step, etc.? 4.2. Q. Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)? (Also phrased as: Can I use a built-in type as base class?) 4.3. Q. Is there a curses/termcap package for Python? 4.4. Q. Is there an equivalent to C's onexit() in Python? 4.5. Q. When I define a function nested inside another function, the nested function seemingly can't access the local variables of the outer function. What is going on? How do I pass local data to a nested function? 4.6. Q. How do I iterate over a sequence in reverse order? 4.7. Q. My program is too slow. How do I speed it up? 4.8. Q. When I have imported a module, then edit it, and import it again (into the same Python process), the changes don't seem to take place. What is going on? 4.9. Q. How do I find the current module name? 4.10. Q. I have a module in which I want to execute some extra code when it is run as a script. How do I find out whether I am running as a script? 4.11. Q. I try to run a program from the Demo directory but it fails with ImportError: No module named ...; what gives? 4.12. Q. I have successfully built Python with STDWIN but it can't find some modules (e.g. stdwinevents). 4.13. Q. What GUI toolkits exist for Python? 4.14. Q. Are there any interfaces to database packages in Python? 4.15. Q. Is it possible to write obfuscated one-liners in Python? 4.16. Q. Is there an equivalent of C's "?:" ternary operator? 4.17. Q. My class defines __del__ but it is not called when I delete the object. 4.18. Q. How do I change the shell environment for programs called using os.popen() or os.system()? Changing os.environ doesn't work. 4.19. Q. What is a class? 4.20. Q. What is a method? 4.21. Q. What is self? 4.22. Q. What is a unbound method? 4.23. Q. How do I call a method defined in a base class from a derived class that overrides it? 4.24. Q. How do I call a method from a base class without using the name of the base class? 4.25. Q. How can I organize my code to make it easier to change the base class? 4.26. Q. How can I find the methods or attributes of an object? 4.27. Q. I can't seem to use os.read() on a pipe created with os.popen(). 4.28. Q. How can I create a stand-alone binary from a Python script? 4.29. Q. Is there a special lib for writing CGI scripts in Python? 4.30. Q. What other WWW tools are there for Python? 4.31. Q. How do I run a subprocess with pipes connected to both input and output? 4.32. Q. How do I call a function if I have the arguments in a tuple? 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. . 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. . (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, . 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. . 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: 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 , 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 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, . The raw archives are also available by ftp, e.g. . 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, is the official Python home page. Michael McLay at NIST maintains a Python page at . And finally the emerging Python Software Association has a preliminary web server at (eventually to be moved to somewhere under python.org). 1.8. Q. Is the Python documentation available on the WWW? A. Yes, see (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. . (This requires some manual patch-up for the references to images, unfortunately.) An Emacs-INFO set containing the library manual is also available by ftp, e.g. . 1.9. Q. Is there a book on Python, or will there be one out soon? A. I am writing one. Publishers are interested. Optimistically, it will be ready for the press by mid-1995. Mark Lutz is working on one too -- more details as they emerge. 1.10. Q. Are there any published articles about Python that I can quote? A. So far the only refereed and published article that describes Python in some detail is: Guido van Rossum and Jelke de Boer, "Interactively Testing Remote Servers Using the Python Programming Language", CWI Quarterly, Volume 4, Issue 4 (December 1991), Amsterdam, pp 283-303. LaTeX source for this paper is available as part of the Python source distribution. 1.11. Q. Are there short introductory papers or talks on Python? A. A recent high-level description of Python is: Guido van Rossum, "An Introduction to Python for UNIX/C Programmers", in the proceedings of the NLUUG najaarsconferentie 1993 (dutch UNIX users group meeting November 1993). PostScript for this paper and for the slides used for the accompanying presentation is available by ftp as and , 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 . 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 . 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 . (I've heard complaints about this service not being very responsive -- try at your own risk.) 1.15. Q. Are there copyright restrictions on the use of Python? A. Hardly. You can do anything you want with the source, as long as you leave the copyrights in, and display those copyrights in any documentation about Python that you produce. Also, don't use the author's institute's name in publicity without prior written permission, and don't hold them responsible for anything (read the actual copyright for a precise legal wording). In particular, if you honor the copyright rules, it's OK to use Python for commercial use, to sell copies of Python in source or binary form, or to sell products that enhance Python or incorporate Python (or part of it) in some form. I would still like to know about all commercial use of Python! 2. Python in the real world =========================== 2.1. Q. How many people are using Python? A. I don't know, but the maximum number of simultaneous subscriptions to the Python mailing list before it was gatewayed into the newsgroup was about 180 (several of which were local redistribution lists). I believe that many active Python users don't bother to subscribe to the list, and now that there's a newsgroup the mailing list subscription is even less meaningful. I see new names on the newsgroup all the time and my best guess is that there are currently at least several thousands of users. 2.2. Q. Have any significant projects been done in Python? A. Here at CWI (the home of Python), we have written a 20,000 line authoring environment for transportable hypermedia presentations, a 5,000 line multimedia teleconferencing tool, as well as many many smaller programs. The University of Virginia uses Python to control a virtual reality engine. Contact: Matt Conway . The ILU project at Xerox PARC can generate Python glue for ILU interfaces. See . The University of California, Irvine uses a student administration system called TELE-Vision written entirely in Python. Contact: Ray Price . 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: . Infoseek uses Python to implement their commercial WWW information retrieval service . Contact: . Michael Powers of daVinci Time & Space is "writing tons-o-python for interactive television entertainment." Contact: . Paul Everitt of Connecting Minds is planning a Lotus Notes gateway. Contact: . Or see their WWW server . 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: (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 . If you'd like to be included in the list above, send me email! 2.4. Q. How stable is Python? A. Very stable. While the current version number would suggest it is in the early stages of development, in fact new, stable releases (numbered 0.9.x through 1.1.x) have been coming out roughly every 3 to 6 months for the past four years. 2.5. Q. When will the next version be released? A. I am planning to release 1.2 in February or early March 1995. It will contain hooks into the implementation of the import command, a (still limited) form of persistent objects, and the usual complement of bug fixes (including many fixed memory leaks and thread problems). 2.6. Q. What new developments are expected for Python in the future? A. There will be better ports to the Mac, DOS, Windows, Windows NT, and OS/2. The Windows ports will support dynamically loaded modules using DLLs. Also planned is improved support for embedding Python in other applications, e.g. by renaming most global symbols to have a "Py" prefix and providing more documentation and threading support. Some proposals were discussed at the recent Python workshop: - persistent objects - safe execution of untrusted Python code - extensions to the import statement for managing packages (groups of related modules) - automatic generation of C/C++ interface glue - interfaces to OMG IDL (== Interface Definition Language by the Object Management Group) - a portable GUI API (Graphical User Interface Application Programmers Interface) - module customization tools - standardized documentation strings on module, class and function objects - the formation of a Python Steering Committee - another Python Workshop For more info, have a look at the WWW page for the last Python Workshop . 2.7. Q. Is it reasonable to propose incompatible changes to Python? A. In general, no. There are already millions of lines of Python code around the world, so any changes in the language that invalidates more than a very small fraction of existing programs has to be frowned upon. Even if you can provide a conversion program, there still is the problem of updating all documentation. Providing a gradual upgrade path is the only way if a feature has to be changed. 3. Building Python ================== 3.1. Q. Is there a test set? A. Yes, simply do "import testall" (or "import autotest" if you aren't interested in the output). The standard modules whose name begins with "test" together comprise the test. The test set doesn't test *all* features of Python but it goes a long way to confirm that a new port is actually working. The Makefile contains an entry "make test" which runs the autotest module. NOTE: if "make test" fails, run the tests manually ("import testall") to see what goes wrong before reporting the error. 3.2. Q. When running the test set, I get complaints about floating point operations, but when playing with floating point operations I cannot find anything wrong with them. A. The test set makes occasional unwarranted assumptions about the semantics of C floating point operations. Until someone donates a better floating point test set, you will have to comment out the offending floating point tests and execute similar tests manually. 3.3. Q. Link errors building Python with STDWIN 0.9.8. on SGI IRIX. A. Get STDWIN 0.9.9 . 3.4. Q. Link errors building Python with STDWIN 0.9.9. A. The linker probably complains that it can't find routines like 'tecreate', 'tenew' etc. The STDWIN 0.9.9 distribution requires that you add TWO libraries from stdwin to the line for stdwin in the Setupfile. Use something like this (all on one line!): stdwin stdwinmodule.c -I$(STDWIN)/H $(STDWIN)/Packs/textedit/libtextedit.a $(STDWIN)/Ports/x11/libstdwin.a -lX11 3.5. Q. Link errors after rerunning the configure script. A. It is generally necessary to run "make clean" after a configuration change. 3.6. Q. The python interpreter complains about options passed to a script (after the script name). A. You are probably linking with GNU getopt, e.g. through -liberty. Don't. The reason for the complaint is that GNU getopt, unlike System V getopt and other getopt implementations, doesn't consider a non-option to be the end of the option list. A quick (and compatible) fix for scripts is to add "--" to the interpreter, like this: #! /usr/local/bin/python -- You can also use this interactively: python -- script.py [options] Note that a working getopt implementation is provided in the Python distribution (in Python/getopt.c) but not automatically used. 3.7. Q. When building on the SGI, make tries to run python to create glmodule.c, but python hasn't been built or installed yet. A. Comment out the line mentioning glmodule.c in Setup and build a python without gl first; install it or make sure it is in your $PATH, then edit the Setup file again to turn on the gl module, and make again. You don't need to do "make clean"; you do need to run "make Makefile" in the Modules subdirectory (or just run "make" at the toplevel). 3.8. Q. Python built with gcc for the DEC Alpha doesn't work. People have reported problems with gcc 2.5.8 up to 2.6.3. The DEC OSF/1 cc compiler does not have these problems so it's likely a gcc bug. The latest news is that this has been fixed in Python 1.2 by a source change (I gave up waiting for a fixed gcc). 3.9. Q. I use VPATH but some targets are built in the source directory. A. On some systems (e.g. Sun), if the target already exists in the source directory, it is created there instead of in the build directory. This is usually because you have previously built without VPATH. Try running "make clobber" in the source directory. 3.10. Q. Trouble building or linking with the GNU readline library. A. Consider using readline 2.0. 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 (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 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). 3.11. 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). 3.12. Q. Trouble with prototypes on Ultrix. A. Ultrix cc seems broken -- use gcc, or edit config.h to #undef HAVE_PROTOTYPES. 3.13. Q. Trouble with posix.listdir on NeXTSTEP 3.2. A. (This is fixed in 1.2.) You need to pass "-posix" to the C compiler, especially in the link phase -- it uses the correct version of the C library. The easiest way is perhaps to do "make OPT=-posix". You also need to remove this NeXT-specific section from import.c: #if defined(NeXT) || defined(WITH_RLD) #define DYNAMIC_LINK #define USE_RLD #endif 3.14. Q. Other trouble building Python on platform X. A. Please email the details to and I'll look into it. Please provide as many details as possible. In particular, if you don't tell me what type of computer and what operating system (and version) you are using it will be difficult for me to figure out what is the matter. If you get a specific error message, please email it to me too. 3.15. Q. How to configure dynamic loading on Lixux. A. (Thanks to Andrew Kuchling): Linux requires that you use the GNU DLD library. The stages of using dynamic libraries on Linux are: 1) Get dld 3.2.5 from a Linux site. Be careful here; the most recent GNU version is 3.2.3, and doesn't support Linux; be sure to get it from a Linux mirror, not a GNU mirror (3.2.4 should also work). Compile it and install the library libdld.a somewhere; I used /usr/local/lib. Suitable URLs for the dld distribution are currently: and . There's also a binary distribution of it: . 2) Get Jack Jansen's DL library; its location is given in the _Extending Python_ manual as ftp://ftp.cwi.nl/pub/dynload/. Compile it and install libdl.a wherever you put libdld.a. 3) Run Python's configure script, giving it the --with-dl-dld option, which requires a parameter giving the directory where you put the libraries. 4) Recompile Python. Note that once ELF is common in the Linux world, it will be much easier (probably within a year) -- the standard SVR4-style dlopen() interface is then available. This has already proved to work perfectly using a beta version of the ELF-capable GCC for Linux. 4. Programming in Python ======================== 4.1. Q. Is there a source code level debugger with breakpoints, step, etc.? A. Yes. Check out module pdb; pdb.help() prints the documentation (or you can read it as Lib/pdb.doc). If you use the STDWIN option, there's also a windowing interface, wdb. You can write your own debugger by using the code for pdb or wdb as an example. 4.2. Q. Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)? (Also phrased as: Can I use a built-in type as base class?) A. No, but you can easily create a Python class which serves as a wrapper around a built-in object, e.g. (for dictionaries): # A user-defined class behaving almost identical # to a built-in dictionary. class UserDict: def __init__(self): self.data = {} def __repr__(self): return repr(self.data) def __cmp__(self, dict): if type(dict) == type(self.data): return cmp(self.data, dict) else: return cmp(self.data, dict.data) def __len__(self): return len(self.data) def __getitem__(self, key): return self.data[key] def __setitem__(self, key, item): self.data[key] = item def __delitem__(self, key): del self.data[key] def keys(self): return self.data.keys() def items(self): return self.data.items() def values(self): return self.data.values() def has_key(self, key): return self.data.has_key(key) 4.3. Q. Is there a curses/termcap package for Python? A. Yes -- Lance Ellinghaus has written a module that interfaces to System V's "ncurses". If you know a little curses and some Python, it's straightforward to use. It is part of the standard Python distribution, but not configured by default -- you must enable it by editing Modules/Setup. It requires a System V curses implementation. You could also consider using the "alfa" (== character cell) version of STDWIN. (Standard Window System Interface, a portable windowing system interface by myself .) 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: 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] 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): Unfortunately, this solution is slowest of all, due to the method call overhead... 4.7. Q. My program is too slow. How do I speed it up? A. That's a tough one, in general. There are many tricks to speed up Python code; I would consider rewriting parts in C only as a last resort. One thing to notice is that function and (especially) method calls are rather expensive; if you have designed a purely OO interface with lots of tiny functions that don't do much more than get or set an instance variable or call another method, you may consider using a more direct way, e.g. directly accessing instance variables. Also see the standard module "profile" (described in the file "python/lib/profile.doc") which makes it possible to find out where your program is spending most of its time (if you have some patience -- the profiling itself can slow your program down by an order of magnitude). 4.8. Q. When I have imported a module, then edit it, and import it again (into the same Python process), the changes don't seem to take place. What is going on? A. For reasons of efficiency as well as consistency, Python only reads the module file on the first time a module is imported. (Otherwise a program consisting of many modules, each of which imports the same basic module, would read the basic module over and over again.) To force rereading of a changed module, do this: import modname reload(modname) Warning: this technique is not 100% fool-proof. In particular, modules containing statements like from modname import some_objects will continue to work with the old version of the imported objects. 4.9. Q. How do I find the current module name? A. A module can find out its own module name by looking at the (predefined) global variable __name__. If this has the value '__main__' you are running as a script. 4.10. Q. I have a module in which I want to execute some extra code when it is run as a script. How do I find out whether I am running as a script? A. See the previous question. E.g. if you put the following on the last line of your module, main() is called only when your module is running as a script: if __name__ == '__main__': main() 4.11. Q. I try to run a program from the Demo directory but it fails with ImportError: No module named ...; what gives? A. This is probably an optional module (written in C!) which hasn't been configured on your system. This especially happens with modules like "Tkinter", "stdwin", "gl", "Xt" or "Xm". For Tkinter, STDWIN and many other modules, see Modules/Setup.in for info on how to add these modules to your Python, if it is possible at all. Sometimes you will have to ftp and build another package first (e.g. STDWIN). Sometimes the module only works on specific platforms (e.g. gl only works on SGI machines). NOTE: if the complaint is about "Tkinter" (upper case T) and you have already configured module "tkinter" (lower case t), the solution is *not* to rename tkinter to Tkinter or vice versa. There is probably something wrong with your module search path. Check out the value of sys.path. For X-related modules (Xt and Xm) you will have to do more work: they are currently not part of the standard Python distribution. You will have to ftp the Extensions tar file, e.g. and follow the instructions there. See also the next question. 4.12. Q. I have successfully built Python with STDWIN but it can't find some modules (e.g. stdwinevents). A. There's a subdirectory of the library directory named 'stdwin' which should be in the default module search path. There's a line in Modules/Setup(.in) that you have to enable for this purpose -- unfortunately in the latest release it's not near the other STDWIN-related lines so it's easy to miss it. 4.13. Q. What GUI toolkits exist for Python? A. Depending on what platform(s) you are aiming at, there are several. - There's a neat object-oriented interface to the Tcl/Tk widget set, called Tkinter. As of python 1.1, it is part of the standard Python distribution -- all you need to do is enable it in Modules/Setup (provided you have already installed Tk and Tcl). This is probably the easiest to install and use, and the most complete widget set. It is also very likely that in the future the standard Python GUI API will be based on or at least look very much like the Tkinter interface. For more info about Tk, including pointers to the source, see John Ousterhout's home page . - 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. or gatekeeper.dec.com in pub/misc/stdwin . STDWIN runs under X11 or the Mac; a Windows port has been attempted but I can't seem to get it working. Note that STDWIN is really not powerful enough to implement a modern GUI (no widgets, etc.) and that I don't have the time to maintain or extend it, so you may be better off using Tkinter or the Motif interface, unless you require portability to the Mac (which is also offered by SUIT, by the way -- see below). - For SGI IRIX only, there's an interface to the complete GL (Graphics Library -- low level but very good 3D capabilities) as well as to FORMS (a buttons-and-sliders-etc package built on top of GL by Mark Overmars -- ftp'able from ). - There's an interface to X11, including the Athena and Motif widget sets (and a few individual widgets, like Mosaic's HTML widget and SGI's GL widget) in the Extensions set, which is separately ftp'able . - There's an interface to SUIT, the U of Virginia's Simple User Interface Toolkit; it can be ftp'ed from . A PC binary of Python 1.0.2 compiled with DJGPP and with SUIT support built-in has been made available by Antonio Costa (a self-extracting archive). Note that the UVa people themselves have expressed doubts about SUIT, and are planning to build a Python GUI API based upon Tk (though not necessarily on Tkinter); see . - There's an interface to WAFE, a Tcl interface to the X11 Motif and Athena widget sets. Last I heard about it it was included in the WAFE 1.0 prerelease . - There's an interface to wxWindows. wxWindows is a portable GUI class library written in C++. It supports XView, Motif, MS-Windows as targets. There is some support for Macs and CURSES as well. wxWindows preserves the look and feel of the underlying graphics toolkit. See the wxPython WWW page at . - Python has been mentioned on the "Futurism" subpage of the Fresco home page . "Pesto" is a Python interface to the CORBA dynamic invocation interface, and thus Fresco. A Pesto prototype is running and is currently being packaged up for inclusion in the Fresco snapshot. 4.14. Q. Are there any interfaces to database packages in Python? A. There's an interface to SYBASE by John Redford . There's an interface to metalbase by Lance Ellinghaus ; it is part of the separate Extensions distribution . Anthony Baxter has written an interface to mSQL (mini-SQL). . Tom Culliton has written an Oracle module. Write him to get a copy of the latest version. 4.15. Q. Is it possible to write obfuscated one-liners in Python? A. Yes. See the following three examples, due to Ulf Bartelt: # Primes < 1000 print filter(None,map(lambda y:y*reduce(lambda x,y:x*y!=0, map(lambda x,y=y:y%x,range(2,int(pow(y,0.5)+1))),1),range(2,1000))) # First 10 Fibonacci numbers print map(lambda x,f=lambda x,f:(x<=1) or (f(x-1,f)+f(x-2,f)): f(x,f), range(10)) # Mandelbrot set print (lambda Ru,Ro,Iu,Io,IM,Sx,Sy:reduce(lambda x,y:x+y,map(lambda y, Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,Sy=Sy,L=lambda yc,Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,i=IM, Sx=Sx,Sy=Sy:reduce(lambda x,y:x+y,map(lambda x,xc=Ru,yc=yc,Ru=Ru,Ro=Ro, i=i,Sx=Sx,F=lambda xc,yc,x,y,k,f=lambda xc,yc,x,y,k,f:(k<=0)or (x*x+y*y >=4.0) or 1+f(xc,yc,x*x-y*y+xc,2.0*x*y+yc,k-1,f):f(xc,yc,x,y,k,f):chr( 64+F(Ru+x*(Ro-Ru)/Sx,yc,0,0,i)),range(Sx))):L(Iu+y*(Io-Iu)/Sy),range(Sy ))))(-2.1, 0.7, -1.2, 1.2, 30, 80, 24) # \___ ___/ \___ ___/ | | |__ lines on screen # V V | |______ columns on screen # | | |__________ maximum of "iterations" # | |_________________ range on y axis # |____________________________ range on x axis Don't try this at home, kids! 4.16. Q. Is there an equivalent of C's "?:" ternary operator? A. Not directly. In many cases you can mimic a?b:c with "a and b or c", but there's a flaw: if b is zero (or empty, or None -- anything that tests false) then c will be selected instead. In many cases you can prove by looking at the code that this can't happen (e.g. because b is a constant or has a type that can never be false), but in general this can be a problem. Steve Majewski (or was it Tim Peters?) suggested the following solution: (a and [b] or [c])[0]. Because [b] is a singleton list it is never false, so the wrong path is never taken; then applying [0] to the whole thing gets the b or c that you really wanted. Ugly, but it gets you there in the rare cases where it is really inconvenient to rewrite your code using 'if'. 4.17. Q. My class defines __del__ but it is not called when I delete the object. A. There are several possible reasons for this. - The del statement does not necessarily call __del__ -- it simply decrements the object's reference count, and if this reaches zero __del__ is called. - If your data structures contain circular links (e.g. a tree where each child has a parent pointer and each parent has a list of children) the reference counts will never go back to zero. You'll have to define an explicit close() method which removes those pointers. Please don't ever call __del__ directly -- __del__ should call close() and close() should make sure that it can be called more than once for the same object. - If the object has ever been a local variable (or argument, which is really the same thing) to a function that caught an expression in an except clause, chances are that a reference to the object still exists in that function's stack frame as contained in the stack trace. Normally, deleting (better: assigning None to) sys.exc_traceback will take care of this. If you a stack was printed for an unhandled exception in an interactive interpreter, delete sys.last_traceback instead. - There is code that deletes all objects when the interpreter exits, but if your Python has been configured to support threads, it is not called (because other threads may still be active). You can define your own cleanup function using sys.exitfunc (see question 4.4). - Finally, there are some obscure bugs if your __del__ method does complicated things such as updating dictionaries or lists or references globals. I hope to have fixed these in release 1.2. 4.18. Q. How do I change the shell environment for programs called using os.popen() or os.system()? Changing os.environ doesn't work. A. Modifying the environment passed to subshells was left out of the interpreter because there seemed to be no well-established portable way to do it (in particular, some systems, have putenv(), others have setenv(), and some have none at all). However if all you want is to pass environment variables to the commands run by os.system() or os.popen(), there's a simple solution: prefix the command string with a couple of variable assignments and export statements. The following would be universal for popen: import os from commands import mkarg # nifty routine to add shell quoting def epopen(cmd, mode, env = {}): # env is a dictionary of environment variables prefix = '' for key, value in env.items(): prefix = prefix + '%s=%s\n' % (key, mkarg(value)[1:]) prefix = prefix + 'export %s\n' % key return os.popen(prefix + cmd, mode) 4.19. Q. What is a class? A. A class is the particular object type that is created by executing a class statement. Class objects are used as templates, to create class instance objects, which embody both the data structure and program routines specific to a datatype. 4.20. Q. What is a method? A. A method is a function that you normally call as x.name(arguments...) for some object x. The term is used for methods of classes and class instances as well as for methods of built-in objects. (The latter have a completely different implementation and only share the way their calls look in Python code.) Methods of classes (and class instances) are defined as functions inside the class definition. 4.21. Q. What is self? A. Self is merely a conventional name for the first argument of a method -- i.e. a function defined inside a class definition. A method defined as meth(self, a, b, c) should be called as x.meth(a, b, c) for some instance x of the class in which the definition occurs; the called method will think it is called as meth(x, a, b, c). 4.22. Q. What is a unbound method? A. An unbound method is a method defined in a class that is not yet bound to an instance. You get an unbound method if you ask for a class attribute that happens to be a function. You get a bound method if you ask for an instance attribute. A bound method knows which instance it belongs to and calling it supplies the instance automatically; an unbound method only knows which class it wants for its first argument (a derived class is also OK). Calling an unbound method doesn't "magically" derive the first argument from the context -- you have to provide it explicitly. 4.23. Q. How do I call a method defined in a base class from a derived class that overrides it? A. If your class definition starts with "class Derived(Base): ..." then you can call method meth defined in Base (or one of Base's base classes) as Base.meth(self, arguments...). Here, Base.meth is an unbound method (see previous question). 4.24. Q. How do I call a method from a base class without using the name of the base class? A. DON'T DO THIS. REALLY. I MEAN IT. It appears that you could call self.__class__.__bases__[0].meth(self, arguments...) but this fails when a doubly-derived method is derived from your class: for its instances, self.__class__.__bases__[0] is your class, not its base class -- so (assuming you are doing this from within Derived.meth) you would start a recursive call. 4.25. Q. How can I organize my code to make it easier to change the base class? A. You could define an alias for the base class, assign the real base class to it before your class definition, and use the alias throughout your class. Then all you have to change is the value assigned to the alias. Incidentally, this trick is also handy if you want to decide dynamically (e.g. depending on availability of resources) which base class to use. Example: BaseAlias = class Derived(BaseAlias): def meth(self): BaseAlias.meth(self) ... 4.26. Q. How can I find the methods or attributes of an object? A. This depends on the object type. For an instance x of a user-defined class, instance attributes are found in the dictionary x.__dict__, and methods and attributes defined by its class are found in x.__class__.__bases__[i].__dict__ (for i in range(len(x.__class__.__bases__))). You'll have to walk the tree of base classes to find *all* class methods and attributes. Many, but not all built-in types define a list of their method names in x.__methods__, and if they have data attributes, their names may be found in x.__members__. However this is only a convention. For more information, read the source of the standard (but undocumented) module newdir. 4.27. Q. I can't seem to use os.read() on a pipe created with os.popen(). A. os.read() is a low-level function which takes a file descriptor (a small integer). os.popen() creates a high-level file object -- the same type used for sys.std{in,out,err} and returned by the builtin open() function. Thus, to read n bytes from a pipe p created with os.popen(), you need to use p.read(n). 4.28. Q. How can I create a stand-alone binary from a Python script? The demo script "Demo/scripts/freeze.py" does what you want. (It's actually not a demo but a support tool -- there is some extra code in the interpreter to accommodate it.) It requires that you have the Python build tree handy, complete with all the lib*.a files. This works by scanning your source recursively for import statements (both forms) and looking for the modules on the standard Python path as well as in the source directory (for built-in modules). It then "compiles" the modules written in Python to C code (array initializers that can be turned into code objects using the marshal module) and creates a custom-made config file that only contains those built-in modules which are actually used in the program. It then compiles the generated C code and links it with the rest of the Python interpreter to form a self-contained binary which acts exactly like your script. 4.29. Q. Is there a special lib for writing CGI scripts in Python? A. There's documentation and code for a cgi.py module by Michael McLay available from (For the curious: CGI or Common Gateway Interface is the protocol between HTTP servers (WWW servers) and programs/scripts they run to perform queries and other tasks that require returning a dynamically generated document.) 4.30. Q. What other WWW tools are there for Python? A. The standard library has a module urllib, which can retrieve most commonly used URL types (file, ftp, http, gopher). The Demo2/www directory (Demo2 has to be retrieved separately from the Python ftp sites ) contains some (really old) code to parse HTML and to display it. Steve Miale has written a modular WWW browser called Dancer. An alpha version can be FTP'ed from . (There are a few articles about Dancer in the (hyper)mail archive .) 4.31. Q. How do I run a subprocess with pipes connected to both input and output? A. This is really a UNIX question. Also, in general, it is unwise to do so, because you can easily cause a deadlock where the parent process is blocked waiting for output from the child, while the child is blocked waiting for input from the child. This can be caused because the parent expects the child to output more text than it does, or it can be caused by data being stuck in stdio buffers due to lack of flushing. The Python parent can of course explicitly flush the data it sends to the child before it reads any output, but if the child is a naive C program it can easily have been written to never explicitly flush its output, even if it is interactive, since flushing is normally automatic. In many cases, all you really need is to run some data through a command and get the result back. Unless the data is infinite in size, the easiest (and often the most efficient!) way to do this is to write it to a temporary file and run the command with that temporary file as input. The standard module tempfile exports a function mktemp() which generates unique temporary file names. If after reading all of the above you still want to connect two pipes to a subprocess's standard input and output, here's a simple solution, due to Jack Jansen: import os import sys import string MAXFD = 100 # Max number of file descriptors in this system def popen2(cmd): cmd = string.split(cmd) p2cread, p2cwrite = os.pipe() c2pread, c2pwrite = os.pipe() pid = os.fork() if pid == 0: # Child os.close(0) os.close(1) if os.dup(p2cread) <> 0: sys.stderr.write('popen2: bad read dup\n') if os.dup(c2pwrite) <> 1: sys.stderr.write('popen2: bad write dup\n') for i in range(3, MAXFD): try: os.close(i) except: pass try: os.execv(cmd[0], cmd) finally: os._exit(1) os.close(p2cread) tochild = os.fdopen(p2cwrite, 'w') os.close(c2pwrite) fromchild = os.fdopen(c2pread, 'r') return fromchild, tochild Note that many interactive programs (e.g. vi) don't work well with pipes substituted for standard input and output. You will have to use pseudo ttys ("ptys") instead of pipes. There is some undocumented code to use these in the library module pty.py -- I'm afraid you're on your own here. What's *really* needed is a Python interface to Don Libes' expect library -- any takers? 4.32. Q. How do I call a function if I have the arguments in a tuple? A. Use the built-in function apply(). For instance, func(1, 2, 3) is equivalent to args = (1, 2, 3) apply(func, args) Note that func(args) is not the same -- it calls func() with exactly one argument, the tuple args, instead of three arguments, the integers 1, 2 and 3. 5. Extending Python =================== 5.1. Q. Can I create my own functions in C? A. Yes, you can create built-in modules containing functions, variables, exceptions and even new types in C. This is explained in the document "Extending and Embedding the Python Interpreter" (the LaTeX file Doc/ext.tex). Also read the chapter on dynamic loading. 5.2. Q. Can I create my own functions in C++? A. Yes, using the C-compatibility features found in C++. Basically you place extern "C" { ... } around the Python include files and put extern "C" before each function that is going to be called by the Python interpreter. Global or static C++ objects with constructors are probably not a good idea. 5.3. Q. How can I execute arbitrary Python statements from C? A. The highest-level function to do this is run_command() which takes a single string argument which is executed in the context of module __main__ and returns 0 for success and -1 when an exception occurred (including SyntaxError). If you want more control, use run_string(); see the source for run_command() in Python/pythonrun.c. 5.4. Q. How can I evaluate an arbitrary Python expression from C? A. Call the function run_string() from the previous question with the start symbol eval_input; it then parses an expression, evaluates it and returns its value. See exec_eval() in Python/bltinmodule.c. 5.5. Q. How do I extract C values from a Python object? A. That depends on the object's type. If it's a tuple, gettuplesize(o) returns its length and gettupleitem(o, i) returns its i'th item; similar for lists with getlistsize(o) and getlistitem(o, i). For strings, getstringsize(o) returns its length and getstringvalue(o) a pointer to its value (note that Python strings may contain null bytes so strlen() is not safe). To test which type an object is, first make sure it isn't NULL, and then use is_stringobject(o), is_tupleobject(o), is_listobject(o) etc. 5.6. Q. How do I use mkvalue() to create a tuple of arbitrary length? A. You can't. Use t = newtupleobject(n) instead, and fill it with objects using settupleitem(t, i, o) -- note that this "eats" a reference count of o. Similar for lists with newlistobject(n) and setlistitem(l, i, o). Note that you *must* set all the tuple items to some value before you pass the tuple to Python code -- newtupleobject(n) initializes them to NULL, which isn't a valid Python value. 5.7. Q. What happened to mktuple(), featured in an example in the Extensions manual? A. It's a typo, I meant newtupleobject() (see previous question). 5.8. Q. How do I call an object's method from C? A. Here's a function (untested) that might become part of the next release in some form. It uses to allow passing the argument list on to vmkvalue(): object *call_method(object *inst, char *methodname, char *format, ...) { object *method; object *args; object *result; va_list va; method = getattr(inst, methodname); if (method == NULL) return NULL; va_start(va, format); args = vmkvalue(format, va); va_end(va); if (args == NULL) { DECREF(method); return NULL; } result = call_object(method, args); DECREF(method); DECREF(args); return result; } This works for any instance that has methods -- whether built-in or user-defined. You are responsible for eventually DECREF'ing the return value. To call, e.g., a file object's "seek" method with arguments 10, 0 (assuming the file object pointer is "f"): res = call_method(f, "seek", "(OO)", 10, 0); if (res == NULL) { ... an exception occurred ... } else { DECREF(res); } Note that since call_object() *always* wants a tuple for the argument list, to call a function without arguments, pass "()" for the format, and to call a function with one argument, surround the argument in parentheses, e.g. "(i)". (This function and a companion will be available in 1.2 under the names PyEval_CallFunction and PyEval_CallMethod.) 5.9. Q. How do I catch the output from print_error()? A. (Due to Mark Hammond): * in Python code, define an object that supports the "write()" method. FWIW, there seems to be a small problem that requires the 'softspace' attribute to be defined too (fixed in 1.2). * redirect sys.stdout and sys.stderr to this object. * call print_error, or just allow the standard traceback mechanism to work. Then, the output will go wherever your write() method sends it. 5.10. Q. How do I access a module written in Python from C? A. You can get a pointer to the module object as follows: module = import_module(""); If the module hasn't been imported yet (i.e. it is not yet present in sys.modules), this initializes the module; otherwise it simply returns the value of sys.modules[""]. Note that it doesn't enter the module into any namespace -- it only ensures it has been initialized and is stored in sys.modules. You can then access the module's attributes (i.e. any name defined in the module) as follows: attr = getattr(module, ""); Calling setattr(), to assign to variables in the module, also works. 6. Python's design ================== 6.1. Q. Why isn't there a generic copying operation for objects in Python? A. Hmm. Maybe there should be one, but it's difficult to assign a useful meaning to copying of open files, sockets and windows, or recursive data structures. As long as you design all your classes yourself you are of course free to define a standard base class that defines an overridable copying operation for all the objects you care about. (One practical point: it would have to be a built-in function, not a standard method name, since not all built-in object types have methods; e.g. strings, integers and tuples don't.) 6.2. Q. Why isn't there a generic way to implement persistent objects in Python? (Persistent == automatically saved to and restored from disk.) A. Hmm, hmm. Basically for the same reasons as why there is no generic copying operation. A partial solution will appear in release 1.2. This will also provide a partial solution to the problem of a generic copying operation. (Partial because it still won't cope with open files etc.; however it does handle user-defined classes!) 6.3. Q. Why isn't there a switch or case statement in Python? A. You can do this easily enough with a sequence of if... elif... elif... else. There have been some proposals for switch statement syntax, but there is no consensus (yet) on whether and how to do range tests. 6.4. Q. Why does Python use indentation for grouping of statements? A. Basically I believe that using indentation for grouping is extremely elegant and contributes a lot to the clarity of the average Python program. Most people learn to love this feature after a while. Some arguments for it: - Since there are no begin/end brackets there cannot be a disagreement between grouping perceived by the parser and the human reader. I remember long ago seeing a C fragment like this: if (x <= y) x++; y--; z++; and staring a long time at it wondering why y was being decremented even for x > y... (And I wasn't a C newbie then either.) - Since there are no begin/end brackets there can be no conflicting coding styles. In C there are loads of different ways to place the braces (including the choice whether to place braces around single statements in certain cases, for consistency). If you're used to reading (and writing) code that uses one style, you will feel at least slightly uneasy when reading (or being required to write) another style. - Many coding styles place begin/end brackets on a line by themself. This makes programs considerably longer and wastes valuable screen space, making it harder to get a good overview over a program. Ideally, a function should fit on one basic tty screen (say, 20 lines). 20 lines of Python are worth a LOT more than 20 lines of C. This is not solely due to the lack of begin/end brackets (the lack of declarations also helps, and the powerful operations of course), but it certainly helps! 6.5. Q. Why are Python strings immutable? A. There are two advantages. One is performance: knowing that a string is immutable makes it easy to lay it out at construction time -- fixed and unchanging storage requirements. (This is also one of the reasons for the distinction between tuples and lists.) The other is that strings in Python are considered as "elemental" as numbers. No amount of activity will change the value 8 to anything else, and in Python, no amount of activity will change the string "eight" to anything else. (Adapted from Jim Roskind) 6.6. Q. Why don't strings have methods like index() or sort(), like lists? A. Good question. Strings currently don't have methods at all (likewise tuples and numbers). Long ago, it seemed unnecessary to implement any of these functions in C, so a standard library module "string" written in Python was created that performs string related operations. Since then, the cry for performance has moved most of them into the built-in module strop (this is imported by module string, which is still the preferred interface, without loss of performance except during initialization). Some of these functions (e.g. index()) could easily be implemented as string methods instead, but others (e.g. sort()) can't, since their interface prescribes that they modify the object, while strings are immutable (see the previous question). 6.7. Q. Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))? A. Functions are used for those operations that are generic for a group of types and which should work even for objects that don't have methods at all (e.g. numbers, strings, tuples). Also, implementing len(), max(), min() as a built-in function is actually less code than implementing them as methods for each type. One can quibble about individual cases but it's really too late to change such things fundamentally now. 6.8. Q. Why can't I derive a class from built-in types (e.g. lists or files)? A. This is caused by the relatively late addition of (user-defined) classes to the language -- the implementation framework doesn't easily allow it. See the answer to question 4.2 for a work-around. This *may* be fixed in the (distant) future. 6.9. Q. Why must 'self' be declared and used explicitly in method definitions and calls? A. By asking this question you reveal your C++ background. :-) When I added classes, this was (again) the simplest way of implementing methods without too many changes to the interpreter. I borrowed the idea from Modula-3. It turns out to be very useful, for a variety of reasons. First, it makes it more obvious that you are using a method or instance attribute instead of a local variable. Reading "self.x" or "self.meth()" makes it absolutely clear that an instance variable or method is used even if you don't know the class definition by heart. In C++, you can sort of tell by the lack of a local variable declaration (assuming globals are rare or easily recognizable) -- but in Python, there are no local variable declarations, so you'd have to look up the class definition to be sure. Second, it means that no special syntax is necessary if you want to explicitly reference or call the method from a particular class. In C++, if you want to use a method from base class that is overridden in a derived class, you have to use the :: operator -- in Python you can write baseclass.methodname(self, ). This is particularly useful for __init__() methods, and in general in cases where a derived class method wants to extend the base class method of the same name and thus has to call the base class method somehow. Lastly, for instance variables, it solves a syntactic problem with assignment: since local variables in Python are (by definition!) those variables to which a value assigned in a function body (and that aren't explicitly declared global), there has to be some way to tell the interpreter that an assignment was meant to assign to an instance variable instead of to a local variable, and it should preferably be syntactic (for efficiency reasons). C++ does this through declarations, but Python doesn't have declarations and it would be a pity having to introduce them just for this purpose. Using the explicit "self.var" solves this nicely. Similarly, for using instance variables, having to write "self.var" means that references to unqualified names inside a method don't have to search the instance's directories. 6.10. Q. Can't you emulate threads in the interpreter instead of relying on an OS-specific thread implementation? A. Unfortunately, the interpreter pushes at least one C stack frame for each Python stack frame. Also, extensions can call back into Python at almost random moments. Therefore a complete threads implementation requires thread support for C. 6.11. Q. Why can't lambda forms contain statements? A. Python lambda forms cannot contain statements because Python's syntactic framework can't handle statements nested inside expressions. However, in Python, this is not a serious problem. Unlike lambda forms in other languages, where they add functionality, Python lambdas are only a shorthand notation if you're too lazy to define a function. Functions are already first class objects in Python, and can be declared in a local scope. Therefore the only advantage of using a lambda form instead of a locally-defined function is that you'll have to invent a name for the function -- but that's just a local variable to which the function object (which is exactly the same type of object that a lambda form yields) is assigned! 6.12. Q. Why is there no more efficient way of iterating over a dictionary than first constructing the list of keys()? A. Have you tried it? I bet it's fast enough for your purposes! In most cases such a list takes only a few percent of the space occupied by the dictionary -- it needs only 4 bytes (the size of a pointer) per key -- a dictionary costs 8 bytes per key plus between 30 and 70 percent hash table overhead, plus the space for the keys and values -- by necessity all keys are unique objects and a string object (the most common key type) costs at least 18 bytes plus the length of the string. Add to that the values contained in the dictionary, and you see that 4 bytes more per item really isn't that much more memory... A call to dict.keys() makes one fast scan over the dictionary (internally, the iteration function does exist) copying the pointers to the key objects into a pre-allocated list object of the right size. The iteration time isn't lost (since you'll have to iterate anyway -- unless in the majority of cases your loop terminates very prematurely (which I doubt since you're getting the keys in random order). I don't expose the dictionary iteration operation to Python programmers because the dictionary shouldn't be modified during the entire iteration -- if it is, there's a very small chance that the dictionary is reorganized because the hash table becomes too full, and then the iteration may miss some items and see others twice. Exactly because this only occurs rarely, it would lead to hidden bugs in programs: it's easy never to have it happen during test runs if you only insert or delete a few items per iteration -- but your users will surely hit upon it sooner or later. 7. Using Python on non-UNIX platforms ===================================== 7.1. Q. Is there a Mac version of Python? A. Yes, see the "mac" subdirectory of the distribution sites, e.g. . 7.2. Q. Is there a DOS version of Python? A. Yes, see the "pc" subdirectory of the distribution sites, e.g. . 7.3. Q. Is there a Windows 3.1(1) version of Python? A. Yes, also see the "pc" subdirectory of the distribution sites, e.g. . 7.4. Q. Is there a Windows NT version of Python? A. Yes. Mark Hammond has built a full NT port. This supports using DLLs for dynamic loading of Python modules, and includes an interface to the Microsoft Foundation Classes and a Python programming environment using it that's written mostly in Python. See -- most mirrors will also have this. Sam Rushing once announced he knows how to build Python for the Windows NT on the DEC Alpha AXP. Note that currently there is no unified compilation environment for all NT platforms -- hopefully Microsoft will fix this with the release of Visual C++ 2.0. 7.5. Q. Is there a Windows 95 version of Python? A. The Windows NT version might work, otherwise the Windows 3.1(1) version should work (isn't Windows 95 supposed to be backwards compatible?). 7.6. Q. Is there an OS/2 version of Python? A. Yes, also see the "pc" subdirectory of the distribution sites, e.g. . 7.7. Q. Is there a VMS version of Python? A. I think not. This question has been asked on the list several times and I've never seen an affirmative answer. 7.8. Q. What about IBM mainframes, or other esoteric non-UNIX platforms? A. Basically, the same story as for VMS... 7.9. Q. Where are the source or Makefiles for the non-UNIX versions? A. The standard sources can (almost) be used. Additional sources can be found in the platform-specific subdirectories of the distribution. 7.10. Q. What is the status and support for the non-UNIX versions? A. I don't have access to most of these platforms, so in general I am dependent on material submitted by volunteers(*). However I strive to integrate all changes needed to get it to compile on a particular platform back into the standard sources, so porting of the next version to the various non-UNIX platforms should be easy. (*) For the Macintosh, that volunteer is me, with help from Jack Jansen . 7.11. Q. I have a PC version but it appears to be only a binary. Where's the library? A. You still need to copy the files from the distribution directory "python/Lib" to your system. If you don't have the full distribution, you can get the file lib.tar.gz from most ftp sites carrying Python; this is a subset of the distribution containing just those files, e.g. . Once you have installed the library, you need to point sys.path to it. Assuming the library is in C:\misc\python\lib, the following commands will point your Python interpreter to it (note the doubled backslashes -- you can also use single forward slashes instead): >>> import sys >>> sys.path.insert(0, 'C:\\misc\\python\\lib') >>> For a more permanent effect, set the environment variable PYTHONPATH, as follows (talking to a DOS prompt): C> SET PYTHONPATH=C:\misc\python\lib 7.12. Q. Where's the documentation for the Mac or PC version? A. The documentation for the Unix version also applies to the Mac and PC versions. Where applicable, differences are indicated in the text. 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)? A. Use an external editor. On the Mac, BBEdit seems to be a popular no-frills text editor. I work like this: start the interpreter; edit a module file using BBedit; import and test it in the interpreter; edit again in BBedit; then use the built-in function reload() to re-read the imported module; etc. Regarding the same question for the PC, Kurt Wm. Hemr writes: "While anyone with a pulse could certainly figure out how to do the same on MS-Windows, I would recommend the NotGNU Emacs clone for MS-Windows. Not only can you easily resave and "reload()" from Python after making changes, but since WinNot auto-copies to the clipboard any text you select, you can simply select the entire procedure (function) which you changed in WinNot, switch to QWPython, and shift-ins to reenter the changed program unit."