[issue46713] Provide a C implementation of collections.abc.KeysView and friends

[Raymond Hettinger]

Raymond Hettinger <raymond.hettinger at gmail.com> added the comment:

Some thoughts:

* Other than set operations, most of the pure python code in the dict view ABCs are fast pass throughs.  There is no point in rewriting these in C:

    def __contains__(self, key):
        return key in self._mapping

    def __iter__(self):
        yield from self._mapping

    def __len__(self):
        return len(self._mapping)

* For the set operations, the pure python code has only a simple for-loop and contains test.  There isn't much fat to be cut:

    def __ge__(self, other):
        if not isinstance(other, Set):
            return NotImplemented
        if len(self) < len(other):
            return False
        for elem in other:
            if elem not in self:
                return False
        return True

* Possibly the eval-loop overhead can be eliminated with Python by using itertools:

    def __ge__(self, other):
        if not isinstance(other, Set):
            return NotImplemented
        if len(self) < len(other):
            return False
        for elem in filterfalse(self.__contains__, other):
            return False
        return True

* That leaves the question of why the dict views are so much faster (presuming that the posted timings are representative).  I haven't looked in detail, but the first candidate that comes to mind is that dictviews_to_set() has a fast path for exact dicts.  That lets it bypass the mapping proxy and exploit the fast path in PySet_New for exact dicts.  That fast path reuses the stored hash values and exploits knowing that the input has no duplicates.  Off-hand, I don't see how that can generalize to the arbitrary mappings supports by the view ABCs.  

* Summary:  A first look, it doesn't seem like a C rewrite of the view ABCs would bear fruit.

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<https://bugs.python.org/issue46713>
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