Downstream in NumPy we've noticed that a "sequence" object defined as below will hang (infinite __getitem__ calls) if we try to turn it into an array. The same holds in CPython for converting it to a list:

class OneList:

    def __len__(self):
        # this won't be checked by
        # PySequence_Fast and several
        # over C API calls
        return 1

    def __getitem__(self, x):
        # called indefinitely by
        # i.e., PySequence_Fast
        return 1

Just to confirm -- this is intentional / desired behavior:

list(OneList()) should hang in CPython?

related: 
https://github.com/numpy/numpy/issues/8912
https://github.com/numpy/numpy/pull/11815
https://stackoverflow.com/a/43566241/2942522