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

Eric Snow ericsnowcurrently at gmail.com
Thu Jun 7 19:08:44 CEST 2012

On Thu, Jun 7, 2012 at 8:12 AM, Larry Hastings <larry at hastings.org> wrote:

On 06/06/2012 06:00 PM, Nick Coghlan wrote:

On Thu, Jun 7, 2012 at 10:52 AM, Eric Snow <ericsnowcurrently at gmail.com> wrote:

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. [...] +1 I'm missing something here.  Can you give me an example of modifying an existing function object such that its Signature would change?  Decorators implementing a closure with a different signature don't count--they return a new function object.

I doubt there are any but corner cases to demonstrate here. I'd don't presume to say what use there may be in changing a function's state. However, the fact is that a change to any of the following would cause a cached signature to be out of sync:

All of these are at least replaceable, if not mutable. If inspect.signature() is intended for function introspection purposes, I'd expect it's return value (cached on signature or not) to reflect the function and not a previous state of the function. I prefer the idea that f.signature means it was set explicitly. Then there's no doubt as to what it's meant to reflect.

Providing a defined mechanism to declare a public signature is good, but using that mechanism for implicit caching seems like a questionable idea. Even when it is cached, I'd be happier if inspect.signature() returned a copy rather than a direct reference to the original.

I'll say this: if we're going down this road of "don't cache Signature objects", then in the case of signature we should definitely return a copy just for consistency's sakes.  It'd be a miserable implementation if signature() sometimes returned the same object and sometimes returned different but equivalent objects when called multiple times on the same object.

+1

-eric

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