[New-bugs-announce] [issue46684] Expose frozenset._hash classmethod

Joshua Bronson report at bugs.python.org
Tue Feb 8 13:22:32 EST 2022

New submission from Joshua Bronson <jabronson at gmail.com>:

collections.abc.Set provides a _hash() method that includes the following in its docstring:

Note that we don't define __hash__: not all sets are hashable.
But if you define a hashable set type, its __hash__ should
call this function.
We match the algorithm used by the built-in frozenset type.

Because Set._hash() is currently implemented in pure Python, users face having to make a potentially challenging decision between whether to trade off runtime efficiency vs. space efficiency:

>>> hash(frozenset(x))  # Should I use this?
>>> Set._hash(x)        # Or this?

The former requires O(n) memory to create the frozenset, merely to throw it immediately away, but on the other hand gets to use frozenset's __hash__ implementation, which is implemented in C.

The latter requires only O(1) memory, but does not get the performance benefit of using the C implementation of this algorithm.

Why not expose the C implementation via a frozenset._hash() classmethod, and change Set._hash() to merely call that?

Then it would be much clearer that using Set._hash() is always the right answer.

messages: 412856
nosy: jab
priority: normal
severity: normal
status: open
title: Expose frozenset._hash classmethod

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