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.

Sorry, but I think you've missed the point of _HashedSeq.  The hash() is called no more than once per instance, not once per instance creation.

    >>> from functools import _HashedSeq
    >>> from unittest.mock import Mock
    >>> test_tup = 1, 2, 3, "hello", "world"
    >>> hash_func = Mock(return_value=999)
    >>> hs = _HashedSeq(test_tup, hash=hash_func)
    >>> hash(hs)
    999
    >>> hash(hs)
    999
    >>> hash(hs)
    999
    >>> hash_func.call_count
    1

New changeset 52d3cbcf546f by Raymond Hettinger in branch 'default':
Issue #29200: Add test for lru cache only calling __hash__ once
https://hg.python.org/cpython/rev/52d3cbcf546f

To honor your report, I've added a test for this functionality.

New changeset 2e7e91785306 by Raymond Hettinger in branch 'default':
Issue #29200: Fix test to use self.assertEqual instead of py.test style tests
https://hg.python.org/cpython/rev/2e7e91785306

thanks for reply :)

2017年1月8日星期日，Roundup Robot <report@bugs.python.org> 写道：

>
> Roundup Robot added the comment:
>
> New changeset 2e7e91785306 by Raymond Hettinger in branch 'default':
> Issue #29200: Fix test to use self.assertEqual instead of py.test style
> tests
> https://hg.python.org/cpython/rev/2e7e91785306
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org <javascript:;>>
> <http://bugs.python.org/issue29200>
> _______________________________________
>