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Diffstat (limited to 'Doc/reference/datamodel.rst')
| -rw-r--r-- | Doc/reference/datamodel.rst | 24 |
1 files changed, 23 insertions, 1 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 85953ad..3fb0bf6 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1277,7 +1277,29 @@ Basic customization inheritance of :meth:`__hash__` will be blocked, just as if :attr:`__hash__` had been explicitly set to :const:`None`. - See also the :option:`-R` command-line option. + + .. note:: + + Note by default the :meth:`__hash__` values of str, bytes and datetime + objects are "salted" with an unpredictable random value. Although they + remain constant within an individual Python process, they are not + predictable between repeated invocations of Python. + + This is intended to provide protection against a denial-of-service caused + by carefully-chosen inputs that exploit the worst case performance of a + dict insertion, O(n^2) complexity. See + http://www.ocert.org/advisories/ocert-2011-003.html for details. + + Changing hash values affects the order in which keys are retrieved from a + dict. Although Python has never made guarantees about this ordering (and + it typically varies between 32-bit and 64-bit builds), enough real-world + code implicitly relies on this non-guaranteed behavior that the + randomization is disabled by default. + + See also :envvar:`PYTHONHASHSEED`. + + .. versionchanged:: 3.3 + Hash randomization is enabled by default. .. method:: object.__bool__(self) |