Q:

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?

A:

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 "set-like" view rather than a list.

For set-like views, all of the operations defined for the abstract base class collections.abc.Set are available (for example, ==, <, or ^).

