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enable hash randomization by default

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Benjamin Peterson 2012-02-21 16:08:05 -05:00
parent 6ca5a4d49f
commit c9f54cf512
9 changed files with 44 additions and 65 deletions
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24
Doc/reference/datamodel.rst
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@ -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)