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| -rw-r--r-- | Doc/using/cmdline.rst | 33 |
1 files changed, 16 insertions, 17 deletions
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst index b14f370..8864b79 100644 --- a/Doc/using/cmdline.rst +++ b/Doc/using/cmdline.rst @@ -229,23 +229,22 @@ Miscellaneous options .. cmdoption:: -R - Turn on hash randomization, so that 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 - construction, O(n^2) complexity. See + Kept for compatibility. On Python 3.3 and greater, hash randomization is + turned on by default. + + On previous versions of Python, this option turns on hash randomization, + so that the :meth:`__hash__` values of str, bytes and datetime + 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. + + Hash randomization is intended to provide protection against a + denial-of-service caused by carefully-chosen inputs that exploit the worst + case performance of a dict construction, 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`. + :envvar:`PYTHONHASHSEED` allows you to set a fixed value for the hash + seed secret. .. versionadded:: 3.2.3 @@ -486,8 +485,8 @@ These environment variables influence Python's behavior. .. envvar:: PYTHONHASHSEED - If this variable is set to ``random``, a random value is used to seed the - hashes of str, bytes and datetime objects. + If this variable is not set or set to ``random``, a random value is used + to seed the hashes of str, bytes and datetime objects. If :envvar:`PYTHONHASHSEED` is set to an integer value, it is used as a fixed seed for generating the hash() of the types covered by the hash |