[Python-Dev] (no subject)

[Phillip J. Eby]

At 05:34 AM 3/14/05 -0800, Michael Chermside wrote:
>Nice... thanks. But I have to ask: is this really the right set of metadata to
>be updating? Here are a few things that perhaps ought be copied by 
>update_meta:
>
>     f.__name__     (already included)
>     f.__doc__      (already included)
>     f.__dict__     (already included)
>     f.__module__   (probably should include)
>     f.func_code.co_filename     (to match f.__name__, but I'd leave it alone)

Leave __module__ alone, too, unless you want to play havoc with any 
inspection tools looking for the source code.


>there's also the annoying fact that in IDLE (and in some other python-aware
>IDEs) one can see the argument signature for a function as a "tool tip"
>or other hint. Very handy that, but if a decorator is applied then all
>you will see is "func(*args, **kwargs)" which is less than helpful. I'm
>not sure whether this CAN be duplicated... I believe it is generated by
>examining the following:
>
>     f.func_code.co_argcount
>     f.func_code.co_varnames
>     f.func_code.co_flags & 0x4
>     f.func_code.co_flags & 0x8
>
>...and I suspect (experimentation seems to confirm this) that if you mangle
>these then the code object won't work correctly. If anyone's got a
>suggestion for fixing this, I'd love to hear it.

One solution is to have a __signature__ attribute that's purely 
documentary.  That is, modifying it wouldn't change the function's actual 
behavior, so it could be copied with update_meta() but then modified by the 
decorator if need be.  __signature__ would basically be a structure like 
the one returned by inspect.getargspec().  Also, 'instancemethod' would 
have a __signature__ equal to its im_func.__signature__ with the first 
argument dropped off, thus making it easy to introspect wrapper chains.

I discussed this approach with Guido in private e-mail a few months back 
during discussion about an article I was writing for DDJ about 
decorators.  We also discussed something very similar to 'update_meta()', 
but never settled on a name.  Originally he wanted me to PEP the whole 
thing, but he wanted it to include optional type declaration info, so you 
can probably see why I haven't done anything on that yet.  :)

However, if we can define a __signature__ format that allows for type 
declaration, I imagine there'd be little problem with moving forward on it.