New submission from Jiajun Huang:
the class definition:
class _HashedSeq(list):
""" This class guarantees that hash() will be called no more than once
per element. This is important because the lru_cache() will hash
the key multiple times on a cache miss.
"""
__slots__ = 'hashvalue'
def __init__(self, tup, hash=hash):
self[:] = tup
self.hashvalue = hash(tup)
def __hash__(self):
return self.hashvalue
and I've test for it:
In [1]: from functools import _HashedSeq
In [2]: from unittest.mock import Mock
In [3]: test_tup = 1, 2, 3, "hello", "world"
In [4]: hash_func = Mock()
In [5]: _HashedSeq(test_tup, hash=hash_func)
Out[5]: [1, 2, 3, 'hello', 'world']
In [6]: _HashedSeq(test_tup, hash=hash_func)
Out[6]: [1, 2, 3, 'hello', 'world']
In [7]: _HashedSeq(test_tup, hash=hash_func)
Out[7]: [1, 2, 3, 'hello', 'world']
In [8]: hash_func.call_count
Out[8]: 3
the hash function had been called 3 times rather than 1.
----------
components: Library (Lib)
messages: 284949
nosy: Jiajun Huang
priority: normal
severity: normal
status: open
title: is it a bug in `functools._HashedSeq`
versions: Python 3.7
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue29200>
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