[Python-Dev] Updated PEP 362 (Function Signature Object)

[Nick Coghlan]

On Wed, Jun 6, 2012 at 10:26 AM, Brett Cannon <brett at python.org> wrote:
> On behalf of Yury, Larry, Jiwon (wherever he ended up), and myself, here is
> an updated version of PEP 362 to address Guido's earlier comments. Credit
> for most of the update should go to Yury with Larry also helping out.
>
> At this point I need a BDFAP and someone to do a code
> review: http://bugs.python.org/issue15008 .

I suspect I have too many ideas for what I want the PEP to accomplish
to make a good BDFL delegate for this one.

One thing I would like to see this PEP achieve is to improve function
introspection for @functools.wraps decorated functions where the
wrapper uses a (*args, **kwds) signature. To that end, I would prefer
to see it added to the functools module rather than to the inspect
module.

I would also like to see native support for "early binding" validation
of call parameters. The idea will be for interfaces that support "lazy
calls" (such as contextlib.ExitStack, unittest.TestCase.addCleanup) to
detect parameter binding errors when the function is *added*, rather
than when it is called. Such lazy call errors are notoriously hard to
debug, since the error site (where the callback is invoked) often
provides no indication as to where the actual error occurred (that is,
when it was registered with the callback interface).

Specific proposals:

- the goals of the PEP be expanded to include error checking of
parameter binding for delayed calls and improve introspection of
function wrappers that accept arbitrary arguments, rather than the
more nebulous "improve introspection support for functions".

- the main new API be added as "functools.signature" (any necessary
infrastructure from inspect would also migrate to functools as private
implementations. affected inspect APIs would either be updated to use
the signature API, or else to just republish the functools
implementation)

- "functools.update_wrapper" be enhanced to set "wrapper.__signature__
= signature(wrapped)"

- At least contextlib and unittest be updated to use
"functools.signature(f).bind(*args, **kwds)" for the relevant callback
APIs

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia