operator.attrgetter/itemgetter is not supported by signature

Hi there 👋 I’m back with another issue 😄 Here’s a reproducible example:

from operator import attrgetter
from plum import Function
f = Function(lambda x: x)
pf = Function(f)
g = attrgetter('a')  # same holds for operator.itemgetter
g = pf.dispatch(g)
g(0)

ValueError: callable operator.attrgetter('a') is not supported by signature

It might be possible for us to implement a workaround on our side but it would be great if it were handled by plum. My thinking, which isn’t very elegant, is to wrap it in another callable that passes through and to set that callable’s __signature__, which inspect.signature uses as a sort of cache. I haven’t tested that yet.

cc @wesselb

---

For completeness, the following also fails with a different error, but isn’t a use-case for us atm so less crucial:

from operator import attrgetter
from plum import dispatch
f = attrgetter('a')
pf = dispatch(f)

AttributeError: 'operator.attrgetter' object has no attribute '__qualname__'

[seeM]

Many thanks, it’s working well!

[wesselb]

This should now work in v1.5.16!