fix: make copy_function work with keyword-only defaults

[thorwhalen]

At this point, we have:

from boltons.funcutils import copy_function

f = lambda x, *, y=2: x * y

f_copy = copy_function(f)

assert f(21) == 42  # all goes well, but...

import pytest, re

with pytest.raises(TypeError):
    f_copy(21)  # missing 1 required keyword-only argument: 'y'
    

So f_copy isn't really a copy.

By adding

...
    if hasattr(orig, "__kwdefaults__"):
        ret.__kwdefaults__ = orig.__kwdefaults__
...

to the function, we get the desired behavior:

f = lambda x, *, y=2: x * y
f_copy = copy_function(f)
assert f(21) == f_copy(21) == 42 

I also extended the test_copy_function test function to protect this behavior in future evolutions.

[mahmoud]

This is great Thor, thanks for the great report and succinct test + fix. I moved the test into the py3-only funcutils tests and kicked off discussions for dropping py2 support via #339. Will get this out in the next release, hopefully in the next week or so.