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

Eric Snow ericsnowcurrently at gmail.com
Thu Jun 7 02:52:05 CEST 2012

On Wed, Jun 6, 2012 at 11:28 AM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:

Never copy attributes by hand, always use 'functools.wraps'.  It copies 'name', 'qualname', and bunch of other attributes to the decorator object.

We'll probably extend it to copy signature too; then 'signature(decor(f))' will be the same as 'signature(f)'.

Having the signature object stored on a function is useful for the cases like this, where the signature object is explicitly set to differ from the function's actual signature. That's a good reason to have inspect.signature(f) look for f.signature and use it if available.

However, I'm not seeing how the other proposed purpose, as a cache for inspect.signature(), is useful. I'd expect that if someone wants a function's signature, they'd call inspect.signature() for it. If they really need the speed of a cache then they can explicitly assign signature.

Furthermore, using signature as a cache may even cause problems. If the Signature object is cached then any changes to the function will not be reflected in the Signature object. Certainly that's an unlikely case, but it is a real case. If f.signature is set, I'd expect it to be either an explicitly set value or exactly the same as the first time inspect.signature() was called for that function. We could make promises about that and do dynamic updates, etc., but it's not useful enough to go to the trouble. And without the guarantees, I don't think using it as a cache is a good idea. (And like I said, allowing/using an explicitly set f.signature is a good thing).

-eric

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