Question

I’m baffled. I’m trying to make a subclass that doesn’t care about any keyword parameters — just passes them all along as is to the superclass, and explicitly sets the one parameter that is required for the constructor. Here’s a simplified version of my code:

class BaseClass(object):
    def __init__(self, required, optional=None):
        pass

def SubClass(BaseClass):
    def __init__(self, **kwargs):
        super(SubClass, self).__init__(None, **kwargs)

a = SubClass(optional='foo')  # this throws TypeError!?!??

This fails with

leo@loki$ python minimal.py
Traceback (most recent call last):
  File "minimal.py", line 9, in <module>
    a = SubClass(optional='foo')
TypeError: SubClass() got an unexpected keyword argument 'optional'

How can it complain about an unexpected keyword argument when the method has **kwargs?

(Python 2.7.3 on Ubuntu)

Answer 1

def SubClass(BaseClass):

is a function, not a class. There’s no error because BaseClass could be an argument name, and nested functions are allowed. Syntax is fun, isn’t it?

class SubClass(BaseClass):