My first Python program

MRAB python at mrabarnett.plus.com
Tue Oct 12 22:04:53 CEST 2010


On 12/10/2010 20:14, Seebs wrote:
> So, I'm new to Python, though I've got a bit of experience in a few other
> languages.  My overall impressions are pretty mixed, but overall positive;
> it's a reasonably expressive language which has a good mix between staying
> out of my way and taking care of stuff I don't want to waste attention on.
>
> My first project was to replace a shell script with a Python script.  The
> context is a project ("pseudo") which does some really hairy stuff in C.
> Part of what it does involves creating hundreds of stub functions.  The
> existing shell script did this successfully, but wasn't particularly
> fast, and was basically unmaintainable.  So I've redone it in Python.
>
> The input is a list of C function declarations, such as:
> 	int foo(char *s);
> and the output is several files, which include lists of these functions,
> declarations for wrappers for them, and so on.  So that would produce
> something to the effect of:
> 	int foo(char *s) {
> 	   /* various magic */
> 	   int rc = -1;
> 	   /* stuff happens */
> 	   rc = wrap_foo(s);
> 	   /* more magic */
> 	   return rc;
> 	}
>
> Where it gets complicated is that there are, of course, special cases;
> for instance, the wrapper for 'int open(char *path, int mode, int flags)' has
> to know that the flags argument is conditional, and not always provided, so
> it declares open as "int open(char *path, int mode, ...)", then extracts
> flags using a va_list.  Weird stuff ensues.  It's a pretty weird hunk of
> code.
>
> The source in its current form:
>
> http://github.com/wrpseudo/pseudo/blob/master/makewrappers
>
> The underlying task is fairly ugly, and it's my first Python project,
> so the result isn't particularly pretty, but I'd be interested in
> feedback on it.  However, I'm not at all sure whether it's appropriate for
> this group to post 467 lines of code with no question beyond "how am
> I screwing this up?"
>
> At this point, it does everything I want it to do, so the question isn't
> "how can I do this?", but "how should I have done this more idiomatically?"
>
> There's a few quirks; one is that I have to run on whatever Python happens
> to be installed on a variety of systems, from RHEL4 to Fedora 13 or so.
> (It is, at least for now, completely unimportant whether I run on non-Linux
> systems.)  I can't rely on packages or extensions which aren't going to
> be in the base Python install.
>
> Apart from that... I'm interested in feedback.  I'm not expecting that
> this is good or idiomatic Python, but I'd like to learn to write Python
> correctly and expressively, and there's nothing like criticism to improve
> code.  And if people would prefer that I post the code here, I could,
> I just figured that it was a bit large.
>
The code does require Python 2 and the use of except ... as ... requires 
at least version 2.6.


Line 51

The __init__ method should always return None. There's no need to be 
explicit about it, just use a plain "return".


Line 68

Instead of:

     if not section in self.sections:

use:

     if section not in self.sections:


Line 78

This:

     file(self.path, 'w')

will never return None. If it can't open the file then it'll raise an 
exception.

The error message says:

     "Couldn't open %s to read a template."

but it's opening the file for writing.


Line 82

You can't really rely on the destructor __del__ being called.


Line 333

Shouldn't you be checking that the name of the attribute you're setting 
doesn't clash with one of the existing attributes? Are you sure that a 
dict wouldn't be a better idea?


Line 447

The form:

     except ... as ...

is in Python versions >= 2.6, but not earlier.


Line 464

This use of del just deletes the name from the namespace and won't 
necessarily call the __del__ method of the 'source' object. It's better 
to rely on something more explicit like a 'close' method. (If you can't 
be sure which version of Python it'll use then context managers are 
probably out anyway!)



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