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Clarify that hash randomization is on by default

This commit is contained in:
Antoine Pitrou 2012-08-01 14:53:16 +02:00
parent a562ed012d
commit 0209dc1ff6
1 changed files with 15 additions and 16 deletions
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Doc/using/cmdline.rst
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@ -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.
Kept for compatibility. On Python 3.3 and greater, hash randomization is
turned on by default.
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
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