path: root/Misc/FAQ

Subject: FAQ: Python -- an object-oriented language
Newsgroups: comp.lang.misc,comp.answers,news.answers
Followup-to: comp.lang.misc
From: guido@cwi.nl (Guido van Rossum)
Reply-to: guido@cwi.nl (Guido van Rossum)
Approved: news-answers-request@MIT.Edu

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.

Many FAQs, including this one, are available by anonymous ftp from
rtfm.mit.edu [18.70.0.209] in the directory pub/usenet/news.answers.  
The name under which a FAQ is archived appears in the Archive-name line 
at the top of the article.  This FAQ is archived as python-faq/part1.

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 mail-server@rtfm.mit.edu 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. 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.