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While working on a subclass of inspect.Signature, I realized that inspect.signature is treated as the constructor. So subclassing isn't so simple. At first I considered adding an extra parameter to inspect.signature allowing different Signature classes. Not much later it became clear that all that code should simply be in classmethod. I've called it Signature.from_callable. Patch attached.

Agree in general, but you need to update the docs also.

Hi Eric,
I'm not sure why do you want this. Having "Signature.from_callable" does not allow you to change behaviour of 'inspect.signature' function. More over, it creates a confusion about what API should be used - 'inspect.signature' or 'inspect.Signature.from_callable'.

The difference is that inspect.signature is not friendly to Signature
subclasses. Without from_callable you can't customize the behavior in
inspect.signature easily.

> The difference is that inspect.signature is not friendly to Signature
> subclasses. Without from_callable you can't customize the behavior in
> inspect.signature easily.
OK, suppose you have "Signature.from_callable". You then create a subclass
MySignature, and override it. How can you customize 'inspect.signature' to 
use the MySignature class?

Eric,
Moreover, 'Signature.from_function' and newly added 'Signature.from_builtin'
are private API, or implementation detail (they are not part of PEP, not
mentioned in the docs). If at some point it is needed to rewrite Signature 
in C, probably those two won't be a part of a new implementation.

It's not about customizing inspect.signature. It's about customizing the
nice behavior of that function, including what type it returns.

OK, got it now.
Green light from me. Looking through the code, I saw that 'from_builtin' doesn't use the 'Signature._parameter_cls' (as in from_function). That's probably needs to be fixed as well, maybe in a separate issue.

New changeset 8a4e44473fdd by Yury Selivanov in branch 'default':
inspect.Signature: Add 'Signature.from_callable' classmethod. Closes #17373
http://hg.python.org/cpython/rev/8a4e44473fdd 

Thanks, Yury!