PyMapping_Keys(), PyMapping_Values() and PyMapping_Items() can return a list or tuple (because they use PySequence_Fast). But end users of this API often work only with lists and raise an exception if corresponding mapping methods return an instance of other type (actually only for tuples). SeeModules/posixmodule.c,Modules/_winapi.c,Modules/_sre.c. The type is even not checked in the latter file.Old documentation said that PyMapping_Keys(o) is equivalent to the Python expression list(o.keys()). Maybe it is worth to make this true. Make PyMapping_Keys(o) etc always returning a list. This could simplify the code that uses this C API. Since keys() etc usually return a dict view, a list or a generator, but not a tuple, this unlikely will cause performance regression.

I would be happy to write a PR that implements that.However, i am not sure which way is better to construct a list from the returnvalue (an iterable, hopefully) of keys() etc.:- Call PyList_Type() (in each of PyMapping_Keys() etc.) on the iterable, and  overwrite the error message in case it is a TypeError.- Write a helper function iterable_as_list(), which uses PyObject_GetIter() and  PySequence_List(), and call it in each of PyMapping_Keys() etc..  (iterable_as_list() would receive "keys" etc., so that it would raise the  appropriate error message, in case of a TypeError.)ISTM that the first one is simpler, but I am not sure about the performancedifference between them.

PySequence_List() is the simplest way in C. I don't know whether we need special error messages now. But for compatibility keep them.

(for knowledge preservation's sake)Resolving this issue would also resolve#31486.

New changeset0ccc0f6c7495be9043300e22d8f38e6d65e8884f by Serhiy Storchaka (Oren Milman) in branch 'master':bpo-28280: Make PyMapping_Keys(), PyMapping_Values() and PyMapping_Items() always return a list (#3840)https://github.com/python/cpython/commit/0ccc0f6c7495be9043300e22d8f38e6d65e8884f