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class ABC is an "abstract base class". class X is its subclass. There's some work that needs to be done in any subclass of ABC , which is easy to forget or do incorrectly. I'd like ABC.__init__() to help catch such mistakes by either: (1) starting that work, or (2) validating it This impacts whether super().__init__() is called at the start or at the end of X.__init__() . Here's a simplified example for illustration purposes: Suppose every subclass of ABC must have an attribute registry , and it must be a list. ABC.__init__() can either (1) initialize registry or (2) check that it was properly created. Following is the sample code for each approach. Approach 1: initialize in ABC class ABC: def __init__(self): self.registry = [] class X: def __init__(self): super().__init__() # populate self.registry here ... Approach 2: validate in ABC class ABC: class InitializationFailure(Exception): pass def __init__(self): try: if not isinstance(self.registry, list): raise ABC.InitializationError() except AttributeError: raise ABC.InitializationError() class X: def __init__(self): self.registry = [] # populate self.registry here ... super().__init__() Which is a better design?

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