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

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 ^).