How can I access the default value for a keyword argument in Python?

Question

I'm trying to use the __defaults__ attribute of a function object to get default values for arguments. It works for this case:

>>> def test(x, y=1): pass
... 
>>> test.__defaults__
(1,)

but for this it doesn't:

>>> def test(*args, y=1): pass
... 
>>> test.__defaults__
>>> 

Why does test.__defaults__ contain None? How can I get the default value for the parameter in this case?

Answer 1

You created a function with a keyword-only argument; you cannot use y as a positional argument. Keyword-argument defaults are stored in __kwdefaults__:

>>> test.__kwdefaults__
{'y': 1}

Quoting the Callable Types section of the Standard Types hierarchy datamodel documentation:

__kwdefaults__: A dict containing defaults for keyword-only parameters.

You should really use the inspect module Signature object to introspect callables however; it'll take care of details like these by providing you with a rich object instead:

>>> sig = inspect.signature(test)
>>> {name: param.default for name, param in sig.parameters.items()
...  if not param.default is param.empty}
{'y': 1}