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I have a Python 2.7 class (call it Child), that is a child of another class (Parent) that is itself a subclass of dict.

I'm trying to define __iter__ in Child in the hopes that when someone does a dict(child_object) I can control how it is converted to a dict. I must be misunderstanding something though, because it seems the dict() call is bypassing calling __iter__ completely and is instead going to the underlying dict.

I did some research and from the dict() method's docs I see that it may be seeing the object as a mapping first, and therefore using that instead of the iterable's __iter__. Is that the case, and if so, is there a way I can overwrite some method that is being called on the mapping?

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    Can you add a simple example to your question? That'd make it much easier to follow what you mean. Commented Oct 16, 2018 at 13:10
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    Do not inherit from dict, but from collections.UserDict Commented Oct 16, 2018 at 13:12
  • Otherwise special methods won't work, when you override them. Commented Oct 16, 2018 at 13:13

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When you use dict() on a mapping (or use dictionary.update(...) passing in a mapping), then Python will not use __iter__. Python looks for a .keys() method to detect mappings. In that case, if the mapping happens to be a dict or a subclass of dict, then a fast path is picked that copies key-value pairs directly from the underlying C structures. You can't prevent this with custom Python functions.

Put differently, if you must define a custom mapping type that lets you control how dict() copies key-value pairs from it, you should not subclass dict. Implement your own mapping type by subclassing collections.Mapping or collections.MutableMapping or a class from the UserDict module.

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Thanks, that confirms my suspicions. I saw UserDict docs say "The need for this class has been largely supplanted by the ability to subclass directly from dict" so I went for dict.

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