Logged In: YES 
user_id=357491

In response to Jim:

(1) Yes.

(2) I don't want to cache it.  Remember this is not meant to
be used on a per-call basis, but for introspection before
calling the functin.

(3) Felt like using the new 'if' expression and I never
liked using the short-circuit of 'or' for assignment.  I can
change it to an 'if' statement if it really bugs you.

(4) Because I didn't want a TypeError that was caused
because of an error in the code to act as an accidental
signal that the check won't work.  I added a comment about that.

(5) I don't quite follow what your problem is here.  Can you
give me an example function def and call that you think is a
problem?

(6) No, that will be added when function
annotations/tags/whatever get added.  No need to prematurely
optimize.

(7) If it fails it should raise an exception, just like it
does now.  If you don't want it stored on the object, call
Signature's constructor directly.