-- * ----------------------------------------------------------------------- * * Martin Sckopke (ecco@ixp2.rz.uni-mannheim.de) * * ----------------------------------------------------------------------- *
Archive-name: python-faq/part1 Version: 1.2 Last-modified: 24 Jan 1994
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, 1994 Guido van Rossum. Unchanged electronic redistribution of this FAQ is allowed. Printed redistribution only with permission of the author. No warranties.
Author's address: Guido van Rossum CWI, dept. CST Kruislaan 413 P.O. Box 94079 1090 GB Amsterdam The Netherlands Email: guido@cwi.nl
The latest version of this FAQ is available by anonymous ftp from ftp.cwi.nl [192.16.184.180] in the directory /pub/python, with filename python-FAQ. It will also be posted regularly to the newsgroups comp.answers and comp.lang.misc.
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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. Is there a newsgroup or mailing list devoted to Python? 1.6. Q. Is there a book on Python, or will there be one out soon? 1.7. Q. Are there any published articles about Python that I can quote?
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. What new developments are expected for Python in the future? 2.5. Q. How stable is Python? 2.6. Q. Any more future plans?
3. Building Python 3.1. Q. I have trouble building the md5 module and/or finding the file md5.c. 3.2. Q. Is there a test set? 3.3. 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. 3.4. Q. I get an OverflowError on evaluating 2*2. What is going on? 3.5. Q. Trouble building Python 0.9.9 on platform X.
4. Programming in Python 4.1. 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?) 4.2. Q. I assign to a variable in a call to exec() but when I try to use it on the next line I get an error. What is going on? 4.3. Q. Why does that work? 4.4. Q. Is there a curses/termcap package for Python? 4.5. Q. Is there an equivalent to C's onexit() in Python? 4.6. 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? 4.7. Q. How do I iterate over a sequence in reverse order? 4.8. Q. My program is too slow. How do I speed it up? 4.9. 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?
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++?
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?
7. Using Python on non-UNIX platforms 7.1. Q. Where's the DOS version of 0.9.9? 7.2. Q. Is there a Windows version of Python? 7.3. Q. I have the Mac or DOS version but it appears to be only a binary. Where's the library? 7.4. Q. Where's the documentation for the Mac or DOS version? 7.5. Q. The Mac 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?
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 MS-DOS.
To find out more, the best thing to do is to start reading the tutorial from the documentation set (see a few questions further down).
1.2. Q. Why is it called Python?
A. Apart from being a computer wizard, I'm also a fan of "Monty Python's Flying Circus" (a BBC comedy series from the seventies, in 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!
1.3. Q. How do I obtain a copy of the Python source?
A. The latest Python source distribution is always available by anonymous ftp from ftp.cwi.nl [192.16.184.180] in the directory /pub/python, with filename python<version>.tar.Z. It is a compressed 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. Currently <version> is 0.9.9. (See section 7 for non-UNIX information.)
1.4. Q. How do I get documentation on Python?
A. The latest Python documentation set is always available by anonymous ftp from ftp.cwi.nl [192.16.184.180] in the directory /pub/python, with filename pythondoc-ps<version>.tar.Z. It is a compressed tar file containing PostScript files of the reference manual, the library manual, and the tutorial. Currently <version> is 0.9.9. (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.
The following sites keep mirrors of the Python distribution:
Site IP address Directory
gatekeeper.dec.com 16.1.0.2 /pub/plan/python/cwi ftp.uu.net 192.48.96.9 /languages/python ftp.wustl.edu 128.252.135.4 /graphics/graphics/sgi-stuff/python ftp.funet.fi 128.214.6.100 /pub/languages/python (old?) ftp.fu-berlin.de 130.133.4.50 /pub/unix/languages/python (python* only)
Or try archie on e.g. python0.9.9.tar.Z to locate the nearest copy of that version...
1.5. Q. Is there a newsgroup or mailing list devoted to Python?
A. There is no Python newsgroup yet; if you want to post to the net about Python, use comp.lang.misc. There is a mailing list devoted to Python; send e-mail to python-list-request@cwi.nl to (un)subscribe. There are plans to start the discussion about creation of comp.lang.python as soon as version 1.0.0 has been released.
1.6. Q. Is there a book on Python, or will there be one out soon?
A. Unfortunately, not yet. I would like to write one but my obligations at CWI include too much other work to make much progress on it. Several parties have expressed interest in sponsoring or helping the production of a book or reference manual, but so far there are no firm plans. If you volunteer help, by all means drop me a note!
1.7. 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.
A more 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 can be found in the ftp directory mentioned a few questions earlier, with filenames nluug-paper.ps and nluug-slides.ps, respectively.
2. Python in the real world ===========================
2.1. Q. How many people are using Python?
A. I don't know, but at the last count there were at least 130 addresses on the Python mailing list (several of which are local redistribution lists). I suspect that many users don't bother to subscribe to the list.
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 multimedia teleconferencing tool, as well as many smaller programs.
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 mailing list).
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>.
2.4. Q. What new developments are expected for Python in the future?
A. I am almost ready to release version 1.0.0 -- it should be out by the end of January 1994. It will have some new functionality and bugfixes and be portable to more platforms. The directory tree structure and build procedure will be radically different -- almost all configuration is now done automatically, using GNU autoconf. User-visible changes include: double-quoted strings, functional programming operations (lambda, map, filter, reduce -- all evaluated eagerly), exec becomes a statement, str() is customizable through __str__ (used by print). The originally planned grand renaming scheme will not be implemented because of lack of time. A beta version can be ftp'ed from the usual sites, file python1.0.0beta.tar.Z.
2.5. Q. How stable is Python?
A. Very stable. While the current version number (0.9.9) would suggest it is in the early stages of development, in fact new, stable releases have been coming out every 3-6 months for the past three years.
2.6. Q. Any more future plans?
A. Without warranty that any of this will actually be realized: I am currently thinking about mechanisms for built-in on-line help and a switch/case statement. There are also some people (independently) working on a windowing interface based on STDWIN but with the power and ease of use of the average modern widget set. I still hope to get some help in producing a Windows version. It would be nice if there were a window-based class browser (Someone at CWI has contributed one using Motif but it needs some work).
3. Building Python ==================
3.1. Q. I have trouble building the md5 module and/or finding the file md5.c.
A. Apparently the md5 module was based on an older version of RSA's md5 implementation. The ftp site rsa.com mentioned in the Makefile where this version was found is no longer accessible, and the version from RFC 1321 (md5c.c) is slightly different. This will be fixed in the 1.0 release; write me if you need the fixes now.
3.2. 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.
3.3. 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.
3.4. Q. I get an OverflowError on evaluating 2*2. What is going on?
A. Your machine probably has 64 bit long integers (e.g. DEC alpha or HP snake architectures). There are some dependencies on word length in file intobject.c. This will be corrected in the 1.0 release; until then, on a 64 bit machine, just comment out the check for overflow from int_mul:
#if 0 if (x > 0x7fffffff || x < (double) (long) 0x80000000) return err_ovf("integer multiplication"); #endif
You should also include <limits.h> and replace the constant 32 by LONG_BIT in int_[lr]shift.
3.5. Q. Trouble building Python 0.9.9 on platform X.
A. In the bootstrap phase (before you have built the first running interpreter), make sure the -D settings in the Makefile are correct for your system. In particular you may have to add or delete -DSYSV. It may also be necessary to change the flags used to compile posixmodule.c and timemodule.c; e.g. on AIX the following are necessary: posixmodule.c: -DHAVE_STDLIB -DNOALTTZ -DOLDTZ -Dunix -DSYSV -DDO_TIMES timemodule.c: -DHAVE_STDLIB -DNOALTTZ -DOLDTZ -Uunix -DSYSV -DBSD_TIME (Note the -Uunix for timemodule!) Those switches for timemodule also require that the #ifdef unix #ifdef BSD_TIME just above: static long millitimer() ( and below the "#endif /* macintosh */" version of millitimer be changed to: #if defined(unix) | defined(BSD_TIME) #ifdef BSD_TIME
4. Programming in Python ========================
4.1. 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)
4.2. Q. I assign to a variable in a call to exec() but when I try to use it on the next line I get an error. What is going on?
A. The reason why this occurs is too complicated to explain (but see the next question). To fix it is easy, however: simply assign None to the variable *before* calling exec(). This will be fixed in the 1.0 release.
4.3. Q. Why does that work?
A. When parsing your program and converting it into internal pseudo code, the interpreter does some optimizations to speed up function execution: it figures out the names of all the local variables and treats them specially. Because your assignment is done by exec(), it is not seen initially by the parser and the variable is not recognized as a local variable. The default treatment is as a global variable, but the exec() statement places it in the local scope, where it is not found. This will be fixed in release 1.0 by making exec into a statement; the parser will then be able to switch off the optimizations for local variables if it encounters an exec statement (recognizing calls to built-in functions is not possible for the parser, hence the syntax change to a statement).
4.4. Q. Is there a curses/termcap package for Python?
A. No, but you can use the "alfa" (== character cell) version of STDWIN. (STDWIN == Standard Windows, a portable windowing system interface by the same author, URL ftp://ftp.cwi.nl/pub/stdwin.) This will also prepare your program for porting to windowing environments such as X11 or the Macintosh.
4.5. 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.
4.6. 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
4.7. Q. How do I iterate over a sequence in reverse order?
A. If it is a list, the fastest solution is
list.reverse() try: for x in list: "do something with x" finally: list.reverse()
This has the disadvantage that while you are in the loop, the list is temporarily reversed. If you don't like this, you can make a copy. This appears expensive but is actually faster than other solutions:
rev = list[:] rev.reverse() for x in rev: <do something with x>
If it isn't a list, a more general but slower solution is:
i = len(list) while i > 0: i = i-1 x = list[i] <do something with x>
A more elegant solution, is to define a class which acts as a sequence and yields the elements in reverse order (solution due to Steve Majewski):
class Rev: def __init__(self, seq): self.forw = seq def __len__(self): return len(self.forw) def __getitem__(self, i): return self.forw[-(i + 1)]
You can now simply write:
for x in Rev(list): <do something with x>
Unfortunately, this solution is slowest of all, due the the method call overhead...
4.8. 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).
4.9. 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 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.
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 all explained in the file "python/misc/EXTENDING". Also read the file "DYNLOAD" there for hints on how to load such extension modules
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.
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.
7. Using Python on non-UNIX platforms =====================================
7.1. Q. Where's the DOS version of 0.9.9?
A. I hope it will be coming soon. A friend with a DOS machine and a compiler has volunteered to build it but he's very busy. Until then, you will have to make do with the 0.9.8 version (which isn't so bad, actually).
7.2. Q. Is there a Windows version of Python?
A. Not yet. Several Windows hackers with C compilers are working on a port though, so maybe we'll have one soon.
7.3. Q. I have the Mac or DOS 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 ftp the file pythonlib0.9.9.tar.Z from site ftp.cwi.nl, directory /pub/python; this is a subset of the distribution containing just those file.
7.4. Q. Where's the documentation for the Mac or DOS version?
A. There isn't any. The documentation for the Unix version also applies to the Mac and DOS versions. Where applicable, differences are indicated in the text.
7.5. Q. The Mac 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?
A. Use an external editor. 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.