Why doesn't dynamically formatting docstrings work? Is there an acceptable workaround for doing this at function definition time?

>>> DEFAULT_BAR = "moe's tavern"
>>> def foo(bar=DEFAULT_BAR):
...     """
...     hello this is the docstring
...     
...     Args:
...       bar (str)    the bar argument (default: {})
...     """.format(DEFAULT_BAR)
...     pass
... 
>>> foo.__doc__
>>> foo.__doc__ is None
True

I tried with old-skool style %s formatting and that didn't work either.

Why doesn't dynamically formatting docstrings work? Is there an acceptable workaround for doing this at function definition time?

>>> DEFAULT_BAR = "moe's tavern"
>>> def foo(bar=DEFAULT_BAR):
...     """
...     hello this is the docstring
...     
...     Args:
...       bar (str)    the bar argument (default: {})
...     """.format(DEFAULT_BAR)
... 
>>> foo.__doc__
>>> foo.__doc__ is None
True

I tried with old-skool style %s formatting and that didn't work either.