Followup-to: comp.lang.python
From: guido@cwi.nl (Guido van Rossum)
Reply-to: guido@cnri.reston.va.us (Guido van Rossum)
-Expires: Fri, 1 Sep 1995 00:00:00 GMT
+Expires: Sun, 1 Oct 1995 00:00:00 GMT
Supersedes: <DCA29p.A9A@cwi.nl>
Approved: news-answers-request@MIT.Edu
Archive-name: python-faq/part1
Submitted-by: Guido van Rossum <guido@cwi.nl>
-Version: 1.25++
-Last-modified: 25 July 1995
+Version: 1.27
+Last-modified: 28 August 1995
This article contains answers to Frequently Asked Questions about
Python (an object-oriented interpreted programming language -- see
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>
+ Guido van Rossum
+ C.N.R.I.
+ 1895 Preston White Drive
+ Reston, VA 22091
+ U.S.A.
+Email: <guido@python.org>, <guido@cnri.reston.va.us>, <guido@cwi.nl>
The latest version of this FAQ is available by anonymous ftp from
<URL:ftp://ftp.python.org/pub/python/FAQ>. It will also be posted
1. General information and availability
2. Python in the real world
- 3. Building Python
+ 3. Building Python and Other Known Bugs
4. Programming in Python
5. Extending Python
6. Python's design
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?
+ 2.8. Q. What is the future of Python?
- 3. Building Python
+ 3. Building Python and Other Known Bugs
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
3.13. Q. How to configure dynamic loading on Lixux.
3.14. Q. Under Solaris 2.x, using GCC, how do I use shared libraries?
3.15. Q. Errors when linking with a shared library containing C++ code.
+ 3.16. Q. Tk menus don't work properly in Python 1.2
+ 3.17. Q. I built with tkintermodule.c enabled but get "Tkinter not found"
+ 3.18. Q. I built with Tk 4.0 but Tkinter complains about the Tk version
+ 3.19. Q. Tk doesn't work right on DEC Alpha
4. Programming in Python
4.1. Q. Is there a source code level debugger with breakpoints, step,
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?
+ 5.11. Q. How do I interface to C++ objects from Python?
6. Python's design
- 6.1. Q. Why isn't there a generic copying operation for objects in
- Python?
+ 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.)
6.11. 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()?
+ 6.13. Q. Can Python be compiled to machine code, C or some other language?
7. Using Python on non-UNIX platforms
7.1. Q. Is there a Mac version of Python?
1. General information and availability
-=======================================
+ =======================================
1.1. Q. What is Python?
A. The following anonymous ftp sites keep mirrors of the Python
distribution:
- <URL:ftp://ftp.cwi.nl/pub/python/>
+ <URL:ftp://ftp.cwi.nl/pub/python/>
<URL:ftp://gatekeeper.dec.com/pub/plan/python/>
<URL:ftp://ftp.uu.net/languages/python/>
<URL:ftp://ftp.wustl.edu/graphics/graphics/sgi-stuff/python/>
<URL:ftp://ftp.sunet.se/pub/lang/python/>
<URL:ftp://unix.hensa.ac.uk/mirrors/uunet/languages/python/>
<URL:ftp://ftp.sterling.com/programming/languages/python/>
- <URL:ftp://ftp.ibp.fr/pub/python/>
+ <URL:ftp://ftp.ibp.fr/pub/python/>
<URL:ftp://ftp.switch.ch/software/sources/python/>
<URL:ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/python/>
Missing-him-too-ly yours...
+PS: support for Python's Emacs mode (Misc/python-mode.el in the
+distribution) has been taken up by Barry Warsaw. Questions about it
+should be mailed to <python-mode@python.org>.
+
2. Python in the real world
===========================
time and my best guess is that there are currently at least several
thousands of users.
+Another statistic is the number of accesses to the Python WWW server.
+Have a look at <URL:http://www.python.org/stats/>.
+
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
system called TELE-Vision written entirely in Python. Contact: Ray
Price <rlprice@uci.edu>.
+See also the next question.
+
If you have done a significant project in Python that you'd like to be
included in the list above, send me email!
development or for as yet unannounced products (witness their
contributions to the Python mailing list or newsgroup).
+SGI has advertised in the Python list looking for Python programmers
+for a project involving interactive television.
+
+See also the workshop minutes at
+<URL:http://www.python.org/workshops/> -- in general the WWW server is
+more up to date than the FAQ for these issues.
+
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:
2.5. Q. When will the next version be released?
-A. I am planning to release 1.3 in September or early October 1995.
-It will contain keyword parameters as the most important new language
-feature. Beta versions may be available as early as August.
+A. I am planning to release 1.3 in early October 1995. It will
+contain keyword parameters as the most important new language feature.
+A beta version was made available in August, more beta versions may
+appear.
2.6. Q. What new developments are expected for Python in the future?
A. See my Work-In-Progress web page, currently at
<URL:http://www.python.org:~guido/WIP.html>, and the pages for the
Second Python Workshop (best reached via the Python home page,
-<URL:http://www.python.org/>).
+<URL:http://www.python.org/>). Also follow the newsgroup discussions!
2.7. Q. Is it reasonable to propose incompatible changes to Python?
the problem of updating all documentation. Providing a gradual
upgrade path is the only way if a feature has to be changed.
+2.8. Q. What is the future of Python?
-3. Building Python
-==================
+A. If I knew, I'd be rich :-)
+
+Seriously, the formation of the PSA (Pyton Software Activity, see
+<URL:http://www.python.org/psa/>) ensures some kind of support even in
+the (unlikely! event that I'd be hit by a bus (actually, here in the
+US, a car accident would be more likely :-), were to join a nunnery,
+or would be head-hunted. A large number of Python users have become
+experts at Python programming as well as maintenance of the
+implementation, and would easily fill the vacuum created by my
+disappearance.
+
+In the mean time, I have no plans to disappear -- rather, I am
+committed to improving Python, and my current benefactor, CNRI (see
+<URL:http://www.cnri.reston.va.us>) is just as committed to continue
+its support of Python and the PSA. In fact, we have great plans for
+Python -- we just can't tell yet!
+
+
+3. Building Python and Other Known Bugs
+=======================================
3.1. Q. Is there a test set?
LINKCC in Modules/Makefile to be your C++ compiler. You may have to
edit config.c slightly to make it compilable with C++.
+3.16. Q. Tk menus don't work properly in Python 1.2.
+
+A. There's a bug in Tkinter.py; remove ClassType and InstanceType from
+the definition of CallableTypes near the top of Lib/tkinter/Tkinter.py.
+
+3.17. Q. I built with tkintermodule.c enabled but get "Tkinter not found".
+
+A. Tkinter.py (note: upper case T) lives in a subdirectory of Lib,
+Lib/tkinter. If you are using the default module search path, you
+probably didn't enable the line in the Modules/Setup file defining
+TKPATH; if you use the environment variable PYTHONPATH, you'll have to
+add the proper tkinter subdirectory.
+
+3.18. Q. I built with Tk 4.0 but Tkinter complains about the Tk version.
+
+A. Several things could cause this. You most likely have a Tk 3.6
+installation that wasn't completely eradicated by the Tk 4.0
+installation (which tends to add "4.0" to its installed files). You
+may have the Tk 3.6 support library installed in the place where the
+Tk 4.0 support files should be (default /usr/local/lib/tk/); you may
+have compiled Python with the old tk.h header file (yes, this actually
+compiles!); you may actually have linked with Tk 3.6 even though Tk
+4.0 is also around. Similar for Tcl 7.4 vs. Tcl 7.3.
+
+3.19. Q. Tk doesn't work right on DEC Alpha.
+
+A. You probably compiled either Tcl, Tk or Python with gcc. Don't.
+For this platform, which has 64-bit integers, gcc is known to generate
+broken code. The standard cc (which comes bundled with the OS!)
+works. If you still prefer gcc, at least try recompiling with cc
+before reporting problems to the newsgroup or the author; if this
+fixes the problem, report the bug to the gcc developers instead. (As
+far as we know, there are no problem with gcc on other platforms --
+the instabilities seem to be restricted to the DEC Alpha.) See also
+question 3.6.
+
4. Programming in Python
========================
Calling setattr(), to assign to variables in the module, also works.
+5.11. Q. How do I interface to C++ objects from Python?
+
+A. Depending on your requirements, there are many approaches. Begin
+by reading the "Extending and Embedding" document (Doc/ext.tex, see
+also <URL:http://www.python.org/doc/>). Realize that for the Python
+run-time system, there isn't a whole lot of difference between C and
+C++ -- so the strategy to build a new Python type around a C structure
+(pointer) type will also work for C++ objects.
+
+Automatic generation of interfaces between Python and C++ is still at
+the horizon -- parsing C++ header files requires an almost complete
+C++ parser, and many features aren't easily translated from C++ to
+Python: certain forms of operator oveloading, function overloading
+(best approached by a varargs function which explicitly type-checks
+its arguments), and reference arguments are just a number of features
+that are hard to translate correctly if at all.
+
+The hardest problem is to transparently translate the C++ class
+hierarchy to Python, so that Python programs derive classes from C++
+classes. Given suitable constraints, this may be possible, but it
+would require more space than I have in this FAQ to explain how.
+In any case, you can get quite a bit done without this, using just the
+existing classes from Python.
+
+If this all seems rather daunting, that may be because it is -- C++
+isn't exactly a baby to handle without gloves! However, people have
+accomplished amazing feats of interfacing between Python and C++, and
+a detailed question posted to the Python list is likely to elicit some
+interesting and useful responses.
+
6. Python's design
==================
-6.1. Q. Why isn't there a generic copying operation for objects in
-Python?
+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.)
+A. Historically, there wasn't. However, starting in Python 1.2,
+there's a library module "copy" which supports both shallow and deep
+copying of most common Python objects (though not things like open
+files, sockets or windows), including an extensible mechanism to copy
+class instances.
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!)
+A. Like the previous question, historically, there wasn't. The
+library module "pickle" now solves this in a very general way (though
+you still can't store things like open files, sockests or windows),
+and the library module "shelve" uses pickle and (g)dbm to create
+presistent mappings containing arbitrary Python objects. There are
+some problems with shelve when using gdbm which will be solved in
+Python 1.3.
6.3. Q. Why isn't there a switch or case statement in Python?
only insert or delete a few items per iteration -- but your users will
surely hit upon it sooner or later.
+6.13. Q. Can Python be compiled to machine code, C or some other language?
+
+A. Not easily. Python's high level data types, dynamic typing of
+objects and run-time invocation of the interpreter (using eval() or
+exec) together mean that a "compiled" Python program would probably
+consist mostly of calls into the Python run-time system, even for
+seemingly simple operations like "x+1". Thus, the performance gain
+would probably be minimal.
+
+Internally, Python source code is always translated into a "virtual
+machine code" or "byte code" representation before it is interpreted
+(by the "Python virtual machine" or "bytecode interpreter"). In order
+to avoid the overhead of parsing and translating modules that rarely
+change over and over again, this byte code is written on a file whose
+name ends in ".pyc" whenever a module is parsed (from a file whose
+name ends in ".py"). When the corresponding .py file is changed, it
+is parsed and translated again and the .pyc file is rewritten. There
+is no performance difference once the .pyc file has been loaded (the
+bytecode read from the .pyc file is exactly the same as the bytecode
+created by direct translation). The only difference is that loading
+code from a .pyc file is faster than parsing and translating a .py
+file, so the presence of precompiled .pyc files will generally improve
+start-up time of Python scripts. If desired, the Lib/compileall.py
+module/script can be used to force creation of valid .pyc files for a
+given set of modules.
+
+If you are looking for a way to translate Python programs in order to
+distribute them in binary form, without the need to distribute the
+interpreter and library as well, have a look at the freeze.py script
+in the Tools/freeze directory. This creates a single binary file
+incorporating your program, the Python interpreter, and those parts of
+the Python library that are needed by your program. Of course, the
+resulting binary will only run on the same type of platform as that
+used to create it.
+
+Hints for proper usage of freeze.py:
+
+- the script must be in a file whose name ends in .py
+
+- you must have installed Python fully:
+
+ make install
+ make libinstall
+ make inclinstall
+ make libainstall
+
7. Using Python on non-UNIX platforms
=====================================
7.8. Q. What about IBM mainframes, or other non-UNIX platforms?
-A. I haven't heard about these, except I remember once hearing about
-an attempt at an OS/9 port. If you're interested in any of this, go
-directly to the newsgroup and ask there, you may find exactly what you
-need. For example, a port to MPE/iX 5.0 on HP3000 computers was just
-announced, see <URL:http://www.allegro.com/software/>.
+A. I haven't heard about these, except I remember hearing about an
+OS/9 port and a port to Vxworks (both operating systems for embedded
+systems). If you're interested in any of this, go directly to the
+newsgroup and ask there, you may find exactly what you need. For
+example, a port to MPE/iX 5.0 on HP3000 computers was just announced,
+see <URL:http://www.allegro.com/software/>.
7.9. Q. Where are the source or Makefiles for the non-UNIX versions?
projects / thirdparty / Python / cpython.git / commitdiff
