docs: def get_fn_args(fn, kwargs: dict, prefix: str = None):
      """
      Given function and a dict of kwargs return a dict containing only the args
      used by the function.
  
      If prefix is specified, also search for args that begin with '{prefix}__'.
      Removes prefix in returned dict.
  
      Raises ValueError if a required arg is missing from the kwargs.
  
      Raises ValueError if both prefixed and unprefixed arg are given in kwargs.
  
      >>> from pprint import pprint as p # Use pprint to sort dict keys
      >>> kwargs = {'a': 1, 'b': 2, 'c': 3, 'x__d': 4}
      >>> def foo(a, b=10): return a + b
      >>> p(get_fn_args(foo, kwargs))
      {'a': 1, 'b': 2}
  
      >>> def bar(a, b, d): return a + b + d
      >>> p(get_fn_args(bar, kwargs, prefix='x'))
      {'a': 1, 'b': 2, 'd': 4}
  
      >>> p(get_fn_args(bar, kwargs))
      Traceback (most recent call last):
      ValueError: The following args are missing for the function bar: ['d']
      """
      all_args = get_all_args(fn)
      required_args = get_required_args(fn)
      fn_kwargs = pick_kwargs(kwargs, all_args, prefix)
  
      missing_args = [arg for arg in required_args if arg not in fn_kwargs]
      if missing_args:
          raise ValueError(
              'The following args are missing for the function '
              '{}: {}.'.format(fn.__name__, missing_args)
          )
  
      return fn_kwargs