functools.partial is not usable

[coldfix]

In most cases, the signature of a functools.partial can not be used by ASTorator: arguments with default values are

• present in the signature - even if they were bound as positional arguments (which cannot be reinserted at call time)
• at the original position! - even if there are more positional parameters behind them (which means that default values will be bound to the wrong parameters in the decorated function)
• bound position arguments effectively become keyword-only arguments. This is (1) a concept which is not natively supported in python2 and (2) not reflected in the value of param.kind, so it can't be handled by python3 code either.

Example:

>>> from functools import partial
>>> from inspect import signature
>>> def func(a, b):
...     pass
>>> for k,v in signature(partial(func, a=1)).parameters.items():
...     print('%s: %s=%s' % (v.kind, v.name, v.default) )
POSITIONAL_OR_KEYWORD: a=1
POSITIONAL_OR_KEYWORD: b=<class 'inspect._empty'>

[coldfix]

The position could be fixed by comparing comparing param.default is param.empty.

I see no way to fix the presence of the parameter from the signature alone.

The keyword-only argument issue makes a fix improbable.

[coldfix]

The first bullet point is actually a lie:

>>> from functools import partial
>>> from inspect import signature

>>> def func(a, b):
...     pass

>>> for k,v in signature(partial(func, 1)).parameters.items():
...     print('%s: %s=%s' % (v.kind, v.name, v.default) )
POSITIONAL_OR_KEYWORD: b=<class 'inspect._empty'>

[coldfix]

Just noticed:
Partial keyword arguments can be determined from the signature: The corresponding parameter will have param._partial_kwarg=True.

This should make it possibe to create keyword only versions on python3.
I don't know if there is any good solution on python2, since can be unbound keyword-only parameters.

[coldfix]

I think, a valid solution is to make all functools.partial objects behave like black_magic.decorator.partial objects with the same bindings, i.e. remove the bound parameters and move the positionial-or-keyword parameters to the left.

[coldfix]

Note, not merging this yet, since functools.partial doesn't work with black_magic.decorator.partial yet. This will probably be very messy to implement.

[coveralls]

Coverage remained the same when pulling a31ce30 on functools.partial into dea304a on master.