The problem is that

  func(mandatory_arg1, mandatory_arg2[, optional_arg3[, optinal_arg4]])

is not compatible with the inspect module.

In many case a meaningful default value was added if this is possible. For example the Python default shown in the signature can be set to '', 'utf-8' or 'strict' while the C default value is NULL for performance. If the parameter is upper index in the sequence it can be set to sys.maxsize (Py_SSIZE_T_MAX in C).

This is not always possible. For example there is not default value for dict.pop().