[issue34494] simple "sequence" class ignoring __len__

[Tyler Reddy]

New submission from Tyler Reddy <treddy at dal.ca>:

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

----------
messages: 324037
nosy: treddy
priority: normal
severity: normal
status: open
title: simple "sequence" class ignoring __len__
type: behavior
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34494>
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