[Python-Dev] Python Library Addition: First-class Procedure Signatures

[Brett Cannon]

On Nov 14, 2007 10:30 AM, Isaac Morland <ijmorlan at cs.uwaterloo.ca> wrote:
> For another project (see my previous email on named tuples), I needed to
> represent procedure signatures, and use them to expand arguments into the
> dictionary of values that exists when execution of a procedure starts.  To my
> surprise, this capability didn't seem to be provided by the Python library,
> even though it clearly is present within the Python system somewhere.
>
> So I wrote a Signature class.  Instances of the class represent all the
> information present between the parentheses of a procedure definition.
> Properties are provided to get the information out, and an expand_args method
> can be called to expand arguments into a dictionary.  This expand_args method
> implements (if I've done it right) the argument conversion part of section
> 5.3.4 of the Python Reference Manual (http://docs.python.org/ref/calls.html).
>

As Collin already pointed out, it sounds like you want PEP 362 to get
into the stdlib.  I have not made a big push to try to get my existing
implementation into Python 2.6/3.0, but I plan to at some point.

> I've put the code below, but I wonder if the real solution is just to create an
> interface to already-existing capability?  It occurs to me that the
> implementation is likely to be in the interpreter itself and not written in
> Python.
>

I don't see why a Python implementation is bad.  If you make this
information lazy then it is not such a big deal to have it take a
little bit longer than if it was implemented in C.

> One possible improvement (and I'm not sure it's better, so I'm just putting it
> out there): perhaps expand_args should be renamed to __call__. Then essentially
> a Signature object would be a procedure whose body is just "return locals ()".

__call__ is already used a method name for objects that can be called.

-Brett