Function _get_argspec_for_partial

tensorflow/python/util/tf_inspect.py:150–235

Implements `getargspec` for `functools.partial` objects. Args: obj: The `functools.partial` obeject Returns: An `inspect.ArgSpec` Raises: ValueError: When callable's signature can not be expressed with ArgSpec.

(obj)

def _get_argspec_for_partial(obj):
  """Implements `getargspec` for `functools.partial` objects.

  Args:
    obj: The `functools.partial` obeject
  Returns:
    An `inspect.ArgSpec`
  Raises:
    ValueError: When callable's signature can not be expressed with
      ArgSpec.
  """
  # When callable is a functools.partial object, we construct its ArgSpec with
  # following strategy:
  # - If callable partial contains default value for positional arguments (ie.
  # object.args), then final ArgSpec doesn't contain those positional arguments.
  # - If callable partial contains default value for keyword arguments (ie.
  # object.keywords), then we merge them with wrapped target. Default values
  # from callable partial takes precedence over those from wrapped target.
  #
  # However, there is a case where it is impossible to construct a valid
  # ArgSpec. Python requires arguments that have no default values must be
  # defined before those with default values. ArgSpec structure is only valid
  # when this presumption holds true because default values are expressed as a
  # tuple of values without keywords and they are always assumed to belong to
  # last K arguments where K is number of default values present.
  #
  # Since functools.partial can give default value to any argument, this
  # presumption may no longer hold in some cases. For example:
  #
  # def func(m, n):
  #   return 2 * m + n
  # partialed = functools.partial(func, m=1)
  #
  # This example will result in m having a default value but n doesn't. This is
  # usually not allowed in Python and can not be expressed in ArgSpec correctly.
  #
  # Thus, we must detect cases like this by finding first argument with default
  # value and ensures all following arguments also have default values. When
  # this is not true, a ValueError is raised.

  n_prune_args = len(obj.args)
  partial_keywords = obj.keywords or {}

  args, varargs, keywords, defaults = getargspec(obj.func)

  # Pruning first n_prune_args arguments.
  args = args[n_prune_args:]

  # Partial function may give default value to any argument, therefore length
  # of default value list must be len(args) to allow each argument to
  # potentially be given a default value.
  no_default = object()
  all_defaults = [no_default] * len(args)

  if defaults:
    all_defaults[-len(defaults):] = defaults

  # Fill in default values provided by partial function in all_defaults.