New submission from Ilya Kulakov <kulakov.i...@gmail.com>: Implementation of memoryview's hashing method [1] imposes the following constraints in order to be hashable (per documentation):
> One-dimensional memoryviews of hashable (read-only) types with formats ‘B’, > ‘b’ or ‘c’ are also hashable. The hash is defined as hash(m) == > hash(m.tobytes()): However it's not clear why original type needs to be hashable given that memoryview deals with 1-dimensional read-only bytes representation of an object. Not only it requires the developer to make an extra copy of C-bytes, but also calls __hash__ of a represented object without using the result other than to detect an error. My particular use case involves a memory view of a readonly numpy's ndarray. My view satisfies the following constraints: >>> print(data.format, data.readonly, data.shape, data.c_contiguous) b True (25,) True But nevertheless the hashing fails because ndarray itself is not hashable. Stefan Krah wrote [2]: > Note that memory_hash() raises an error if the exporter *itself* is not hashable, so it only hashes immutable objects by design. But while __hash__ indeed tells that object is (supposed to be) immutable, there is no requirement for all immutable objects to have __hash__. Both threads I have found ([3], [4]) are quite lengthy and show certain discord in opinions regarding the issue. Perhaps after 6 years since the release of the feature the view on the problem has changed? 1: https://github.com/python/cpython/blob/d1e717588728a23d576c4ead775f7dbd68149696/Objects/memoryobject.c#L2829-L2876 2: https://bugs.python.org/issue15814#msg169510 3: https://bugs.python.org/issue15573 4: https://bugs.python.org/issue15573 ---------- components: Library (Lib) messages: 332280 nosy: Kentzo, skrah priority: normal severity: normal status: open title: memoryview needlessly (?) requires represented object to be hashable versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue35548> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
