bpo-37645: simplify __str__ of function objects

[jdemeyer]

CC @vstinner

https://bugs.python.org/issue37645

[epicfaace]

Would repr(f) when f takes arguments also look like f()? In that case, it might be misleading (it would look like f takes no arguments)

[jdemeyer]

I'm not changing repr(f) at all, only str(f). But yes, str(f) will always be f() regardless of the signature. Note that neither repr(f) nor error messages involving f write the signature, so I don't plan to add the signature either.

>>> def f(a, b): pass
>>> f
<function f at 0x7f9c6e259160>
>>> f(**1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() argument after ** must be a mapping, not int

[epicfaace]

Sorry, I meant str(f). My concern was just that printing out f() may cause the misleading impression that f takes no arguments, while that is not the case for <function f at 0x7f9c6e259160> (because it has no parentheses).