Most C API uses type slots instead looking up in the type's dictionary
for methods. But there are few methods used in basic operations which do
not have corresponding slots. For example "keys", "__trunc__" and
"__length_hint__".
In particularly, "keys" is checked in dict.__init__() and dict.update()
with a positional argument to distinguish a mapping from a sequence of
key-value pairs. If there is such method, it is called in
PyDict_Merge(). Actually "keys" is looked up twice in these operations.
PyDict_Merge() is called every time when you have a call like `f(a=1,
**kw)` (keyword arguments with var-keyword arguments). It is optimized
for dicts, but if kw is something different (for example OrderedDict or
mappingproxy) the keys() method is called.
So would not be worth to add slots for keys (and maybe for values and
items) to the tp_as_mapping structure?
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