In Objects/dictobject.c, if I remove the INCREF/DECREF pair around PyObject_RichCompareBool(), all the tests still pass:

-           Py_INCREF(startkey);
            cmp = PyObject_RichCompareBool(startkey, key, Py_EQ);
-           Py_DECREF(startkey);

It would be nice to have tests demonstrating why it is necessary.  IIRC the reason is that an object could have a custom comparison that clears the enclosing container and frees the element during the comparison.  

Also, it would be nice if PyObject_RichCompareBool() could be redesigned to not fail if a borrowed reference gets deallocated while the function is running.  The incref and decref occur on a hot path, and it is unfortunate that they cause extra writes to objects scattered all over memory, likely causing a lot of cache misses in real programs.

See issue1517.

I can crash python interpreter with few lines of code when if we remove those codes.



class S(str):

    def __eq__(self, other):
        d.clear()
        return NotImplemented

    def __hash__(self):
        return hash('test')

d = {S(): 'value'}
'test' in d

Would you like to add a test for this case?

> Would you like to add a test for this case?

Oh, I mean, is it good to add a test for this case?

New changeset 785f5e6d674306052bf865677d885c30561985ae by Dong-hee Na in branch 'master':
bpo-40489: Add test case for dict contain use after free (GH-19906)
https://github.com/python/cpython/commit/785f5e6d674306052bf865677d885c30561985ae

Thank you.