Released version 1.4; out after 1.0.0 is released...
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Guido van Rossum
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Misc/FAQ
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Misc/FAQ
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@ -6,8 +6,8 @@ Reply-to: guido@cwi.nl (Guido van Rossum)
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Approved: news-answers-request@MIT.Edu
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Archive-name: python-faq/part1
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Version: 1.3
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Last-modified: 26 Jan 1994
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Version: 1.4
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Last-modified: 2 Feb 1994
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This article contains answers to Frequently Asked Questions about
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Python (an object-oriented interpreted programming language -- see
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@ -83,26 +83,39 @@ Here's an overview of the questions per chapter:
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find anything wrong with them.
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3.4. Q. I get an OverflowError on evaluating 2*2. What is going on?
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3.5. Q. Trouble building Python 0.9.9 or earlier on platform X.
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3.6. Q. Trouble building Python 1.0.0 on platform X.
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3.6. Q. Link errors building Python with STDWIN on SGI Irix.
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3.7. Q. Link errors for dlopen, dlsym, dlerror from import.o.
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3.8. Q. Link errors after rerunning the configure script.
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3.9. Q. The python interpreter complains about options passed to a
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script (after the script name).
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3.10. Q. When building on the SGI, make tries to run python to create
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glmodule.c, but python hasn't been built or installed yet.
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3.11. Q. Intermittent core dumps and complaints about perfectly valid
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argument lists to built-in functions (e.g. posix.stat(filename) says
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"no arguments needed").
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3.12. Q. Trouble building Python 1.0.0 on NeXT.
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3.13. Q. Other trouble building Python 1.0.0 on platform X.
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4. Programming in Python
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4.1. Q. Can I create an object class with some methods implemented in
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4.1. Q. Is there a source code level debugger with breakpoints, step,
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etc.?
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4.2. Q. Can I create an object class with some methods implemented in
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C and others in Python (e.g. through inheritance)? (Also phrased as:
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Can I use a built-in type as base class?)
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4.2. Q. I assign to a variable in a call to exec() but when I try to
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use it on the next line I get an error. What is going on?
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4.3. Q. Why does that work?
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4.4. Q. Is there a curses/termcap package for Python?
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4.5. Q. Is there an equivalent to C's onexit() in Python?
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4.6. Q. When I define a function nested inside another function, the
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4.3. Q. Is there a curses/termcap package for Python?
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4.4. Q. Is there an equivalent to C's onexit() in Python?
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4.5. Q. When I define a function nested inside another function, the
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nested function seemingly can't access the local variables of the
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outer function. What is going on? How do I pass local data to a
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nested function?
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4.7. Q. How do I iterate over a sequence in reverse order?
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4.8. Q. My program is too slow. How do I speed it up?
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4.9. Q. When I have imported a module, then edit it, and import it
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4.6. Q. How do I iterate over a sequence in reverse order?
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4.7. Q. My program is too slow. How do I speed it up?
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4.8. Q. When I have imported a module, then edit it, and import it
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again (into the same Python process), the changes don't seem to take
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place. What is going on?
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4.9. Q. I have a module in which I want to execute some extra code when it
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is run as a script. How do I find out whether I am running as a
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script?
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5. Extending Python
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5.1. Q. Can I create my own functions in C?
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@ -363,7 +376,54 @@ LONG_BIT in int_[lr]shift.
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Please convert to Python 1.0.0 -- it is much more portable.
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3.6. Q. Trouble building Python 1.0.0 on platform X.
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3.6. Q. Link errors building Python with STDWIN on SGI IRIX.
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A. Rebuild STDWIN, specifying "CC=cc -cckr" in the Makefile.
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3.7. Q. Link errors for dlopen, dlsym, dlerror from import.o.
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A. You are probably using the GNU loader which doesn't understand
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dynamic linking. Manually comment out #define HAVE_DLFCN_H from
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config.h. (Should be fixed in 1.0.1.)
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3.8. Q. Link errors after rerunning the configure script.
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A. It is generally necessary to run "make clean" after a configuration
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change.
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3.9. Q. The python interpreter complains about options passed to a
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script (after the script name).
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A. You are probably linking with GNU getopt, e.g. through -liberty.
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Don't. (If you are using this because you link with -lreadline, use
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the readline distributed with Python instead.)
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3.10. Q. When building on the SGI, make tries to run python to create
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glmodule.c, but python hasn't been built or installed yet.
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A. Comment out the line mentioning glmodule.c in Setup and build a
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python without gl first; install it or make sure it is in your $PATH,
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then edit the Setup file again to turn on the gl module, and make
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again. You don't need to do "make clean"; you do need to run "make
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Makefile" in the Modules subdirectory (or just run "make" at the
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toplevel).
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3.11. Q. Intermittent core dumps and complaints about perfectly valid
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argument lists to built-in functions (e.g. posix.stat(filename) says
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"no arguments needed").
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A. You are probably using <stdarg.h> instead of <varargs.h>, or the
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other way around. It may also be that your <stdarg.h> or <varargs.h>
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does not match the code generated by your compiler. In simple cases,
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it may help to turn comment out the #define HAVE_STDARG_H from the
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generated config.h.
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3.12. Q. Trouble building Python 1.0.0 on NeXT.
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A. Manually add #define _POSIX_SOURCE to config.h. (Should be fixed
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in 1.0.1.)
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3.13. Q. Other trouble building Python 1.0.0 on platform X.
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A. Please email the details to <guido@cwi.nl> and I'll look into it.
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@ -371,7 +431,15 @@ A. Please email the details to <guido@cwi.nl> and I'll look into it.
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4. Programming in Python
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========================
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4.1. Q. Can I create an object class with some methods implemented in
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4.1. Q. Is there a source code level debugger with breakpoints, step,
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etc.?
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A. Yes. Check out module pdb; pdb.help() prints the documentation (or
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you can read it as Lib/pdb.doc). If you use the STDWIN option,
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there's also a windowing interface, wdb. You can write your own
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debugger by using the code for pdb or wdb as an example.
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4.2. Q. Can I create an object class with some methods implemented in
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C and others in Python (e.g. through inheritance)? (Also phrased as:
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Can I use a built-in type as base class?)
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@ -397,30 +465,7 @@ wrapper around a built-in object, e.g. (for dictionaries):
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def values(self): return self.data.values()
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def has_key(self, key): return self.data.has_key(key)
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4.2. Q. I assign to a variable in a call to exec() but when I try to
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use it on the next line I get an error. What is going on?
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A. The reason why this occurs is too complicated to explain (but see
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the next question). To fix it is easy, however: simply assign None to
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the variable *before* calling exec(). This will be fixed in the 1.0
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release.
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4.3. Q. Why does that work?
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A. When parsing your program and converting it into internal pseudo
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code, the interpreter does some optimizations to speed up function
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execution: it figures out the names of all the local variables and
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treats them specially. Because your assignment is done by exec(), it
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is not seen initially by the parser and the variable is not recognized
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as a local variable. The default treatment is as a global variable,
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but the exec() statement places it in the local scope, where it is not
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found. This will be fixed in release 1.0 by making exec into a
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statement; the parser will then be able to switch off the
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optimizations for local variables if it encounters an exec statement
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(recognizing calls to built-in functions is not possible for the
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parser, hence the syntax change to a statement).
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4.4. Q. Is there a curses/termcap package for Python?
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4.3. Q. Is there a curses/termcap package for Python?
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A. No, but you can use the "alfa" (== character cell) version of
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STDWIN. (STDWIN == Standard Windows, a portable windowing system
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@ -428,13 +473,13 @@ interface by the same author, URL ftp://ftp.cwi.nl/pub/stdwin.)
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This will also prepare your program for porting to windowing
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environments such as X11 or the Macintosh.
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4.5. Q. Is there an equivalent to C's onexit() in Python?
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4.4. Q. Is there an equivalent to C's onexit() in Python?
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A. Yes, if you import sys and assign a function to sys.exitfunc, it
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will be called when your program exits, is killed by an unhandled
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exception, or (on UNIX) receives a SIGHUP or SIGTERM signal.
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4.6. Q. When I define a function nested inside another function, the
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4.5. Q. When I define a function nested inside another function, the
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nested function seemingly can't access the local variables of the
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outer function. What is going on? How do I pass local data to a
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nested function?
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@ -457,7 +502,7 @@ method of an instance of that class, e.g.:
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print twice(10)
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# Output: 20
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4.7. Q. How do I iterate over a sequence in reverse order?
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4.6. Q. How do I iterate over a sequence in reverse order?
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A. If it is a list, the fastest solution is
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@ -505,7 +550,7 @@ You can now simply write:
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Unfortunately, this solution is slowest of all, due the the method
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call overhead...
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4.8. Q. My program is too slow. How do I speed it up?
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4.7. Q. My program is too slow. How do I speed it up?
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A. That's a tough one, in general. There are many tricks to speed up
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Python code; I would consider rewriting parts in C only as a last
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@ -520,7 +565,7 @@ your program is spending most of its time (if you have some patience
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-- the profiling itself can slow your program down by an order of
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magnitude).
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4.9. Q. When I have imported a module, then edit it, and import it
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4.8. Q. When I have imported a module, then edit it, and import it
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again (into the same Python process), the changes don't seem to take
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place. What is going on?
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@ -541,6 +586,18 @@ modules containing statements like
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will continue to work with the old version of the objects imported
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thus.
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4.9. Q. I have a module in which I want to execute some extra code when it
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is run as a script. How do I find out whether I am running as a
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script?
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A. A module can find out its own module name by alooking at the
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(predefined) global variable __name__. If this has the value
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'__main__' you are running as a script. E.g. if you put the following
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on the last line of your module, main() is called only when your
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module is running as a script:
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if __name__ == '__main__': main()
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5. Extending Python
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===================
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@ -548,9 +605,9 @@ thus.
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5.1. Q. Can I create my own functions in C?
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A. Yes, you can create built-in modules containing functions,
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variables, exceptions and even new types in C. This is all explained
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in the file "python/misc/EXTENDING". Also read the file "DYNLOAD"
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there for hints on how to load such extension modules
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variables, exceptions and even new types in C. This is explained in
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the document "Extending and Embedding the Python Interpreter" (the
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LaTeX file Doc/ext.tex). Also read the chapter on dynamic loading.
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5.2. Q. Can I create my own functions in C++?
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