[Python-Dev] A Desperate Plea For Introspection (aka: BDFAP Needed)

[Yury Selivanov]

Guido,

On 2012-06-22, at 2:52 PM, Guido van Rossum wrote:

> This looks great, much better than the version I reviewed half a year
> ago! Thanks you and others (especially Yuri) for all your efforts in
> guiding the discussion and implementing as the discussion went along;
> also thanks to Nick for participating to intensely.
> 
> Quick review notes:
> 
> (1) I don't like APIs that require you to use hasattr(), which you use
> for return_annotation. Since you have Signature.empty to remove it,
> why not set it to Signature.empty when not set? Ditto for other values
> that are currently not set when they don't apply (Parameter.empty I
> think.)

I think that if a function lacks an annotation, that should be reflected
in the same way for its signature.

Currently:

    if hasattr(signature, 'return_annotation'):

If we use Signature.empty:

    if signature.return_annotation is not signature.empty:

So (in my humble opinion) it doesn't simplify things too much.
And also you can use 'try .. except AttributeError .. else' blocks,
which make code even more readable.

All in all, I don't have a very strong opinion in this topic.
'empty' will also work.  When python-dev collectively decided to 
go with missing attributes, 'empty' didn't yet exist (we added 
it with 'replace()' methods).

If you think that using 'empty' is better, we can add that to the PEP.

> (2) Could use an example on how to remove and add parameters using replace().

You have to build a new list of parameters and then pass it to 'replace()'.
Example (from the actual signature() function implementation):

    if isinstance(obj, types.MethodType):
        # In this case we skip the first parameter of the underlying
        # function (usually `self` or `cls`).
        sig = signature(obj.__func__)
        return sig.replace(parameters=tuple(sig.parameters.values())[1:])

> (3) You are using name(arg1, *, arg2) a lot. I think in most cases you
> mean for arg2 to be an optional keyword arg, but this notation doesn't
> convey that it is optional. Can you clarify?

Yes, I meant optional.  Would 'name(arg1, *, [arg2])' be better?

> (4) "If the object is a method" -- shouldn't that be "bound method"?
> (Unbound methods are undetectable.) Or is there some wider definition
> of method? What does it do for static or class methods?

Yes, it should be "If the object is a bound method".  We'll fix this
shortly.

classmethod as a descriptor returns a BoundMethod (bound to the class),
staticmethod returns the original unmodified function, so both of
them are supported automatically.

> (5) Too bad there's no proposal for adding signatures to builtin
> functions/methods, but understood.
> 
> Of these, only (1) is a blocker for PEP acceptance -- I'd either like
> to see this defended vigorously (maybe it was discussed? then please
> quote, I can't go back and read the threads) or changed.
> 
> Otherwise it looks great!
> 
> --Guido

Thanks!

-
Yury