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)
When invoking Python, you may specify any of these options::
- python [-bBdEhiORqsSuvVWx?] [-c command | -m module-name | script | - ] [args]
+ python [-bBdEhiOqsSuvVWx?] [-c command | -m module-name | script | - ] [args]
The most common use case is, of course, a simple invocation of a script::
.. envvar:: PYTHONHASHSEED
- If this variable is set to ``random``, the effect is the same as specifying
- the :option:`-R` option: a random value is used to seed the hashes of str,
- bytes and datetime objects.
+ If this variable is 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
values.
The integer must be a decimal number in the range [0,4294967295]. Specifying
- the value 0 will lead to the same hash values as when hash randomization is
- disabled.
+ the value 0 will disable hash randomization.
.. versionadded:: 3.2.3
hashes = []
for i in range(2):
code = 'print(hash("spam"))'
- rc, out, err = assert_python_ok('-R', '-c', code)
+ rc, out, err = assert_python_ok('-c', code)
self.assertEqual(rc, 0)
hashes.append(out)
self.assertNotEqual(hashes[0], hashes[1])
# Verify that sys.flags contains hash_randomization
code = 'import sys; print("random is", sys.flags.hash_randomization)'
- rc, out, err = assert_python_ok('-R', '-c', code)
+ rc, out, err = assert_python_ok('-c', code)
self.assertEqual(rc, 0)
self.assertIn(b'random is 1', out)
else:
known_hash_of_obj = -1600925533
- # Randomization is disabled by default:
- self.assertEqual(self.get_hash(self.repr_), known_hash_of_obj)
+ # Randomization is enabled by default:
+ self.assertNotEqual(self.get_hash(self.repr_), known_hash_of_obj)
# It can also be disabled by setting the seed to 0:
self.assertEqual(self.get_hash(self.repr_, seed=0), known_hash_of_obj)
- Issue #14051: Allow arbitrary attributes to be set of classmethod and
staticmethod.
-- Issue #13703: oCERT-2011-003: add -R command-line option and PYTHONHASHSEED
- environment variable, to provide an opt-in way to protect against denial of
- service attacks due to hash collisions within the dict and set types. Patch
- by David Malcolm, based on work by Victor Stinner.
+- Issue #13703: oCERT-2011-003: Randomize hashes of str and bytes to protect
+ against denial of service attacks due to hash collisions within the dict and
+ set types. Patch by David Malcolm, based on work by Victor Stinner.
- Issue #13020: Fix a reference leak when allocating a structsequence object
fails. Patch by Suman Saha.
.B \-OO
]
[
-.B \-R
-]
-[
.B \-s
]
[
Do not print the version and copyright messages. These messages are
also suppressed in non-interactive mode.
.TP
-.B \-R
-Turn on "hash randomization", so that the hash() values of str, bytes and
-datetime objects are "salted" with an unpredictable pseudo-random value.
-Although they remain constant within an individual Python process, they are
-not predictable between repeated invocations of Python.
-.IP
-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.
-.TP
.B \-s
Don't add user site directory to sys.path.
.TP
If this is set to a comma-separated string it is equivalent to
specifying the \fB\-W\fP option for each separate value.
.IP PYTHONHASHSEED
-If this variable is set to "random", the effect is the same as specifying
-the \fB-R\fP option: a random value is used to seed the hashes of str,
-bytes and datetime objects.
+If this variable is set to "random", a random value is used to seed the hashes
+of str, bytes and datetime objects.
If 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 randomization. Its
values.
The integer must be a decimal number in the range [0,4294967295]. Specifying
-the value 0 will lead to the same hash values as when hash randomization is
-disabled.
+the value 0 will disable hash randomization.
.SH AUTHOR
The Python Software Foundation: http://www.python.org/psf
.SH INTERNET RESOURCES
-O : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x\n\
-OO : remove doc-strings in addition to the -O optimizations\n\
-q : don't print version and copyright messages on interactive startup\n\
--R : use a pseudo-random salt to make hash() values of various types be\n\
- unpredictable between separate invocations of the interpreter, as\n\
- a defence against denial-of-service attacks\n\
-s : don't add user site directory to sys.path; also PYTHONNOUSERSITE\n\
-S : don't imply 'import site' on initialization\n\
";
"PYTHONFAULTHANDLER: dump the Python traceback on fatal errors.\n\
";
static char *usage_6 = "\
-PYTHONHASHSEED: if this variable is set to ``random``, the effect is the same \n\
- as specifying the :option:`-R` option: a random value is used to seed the\n\
- hashes of str, bytes and datetime objects. It can also be set to an integer\n\
- in the range [0,4294967295] to get hash values with a predictable seed.\n\
+PYTHONHASHSEED: if this variable is set to ``random``, a random value is used\n\
+ to seed the hashes of str, bytes and datetime objects. It can also be\n\
+ set to an integer in the range [0,4294967295] to get hash values with a\n\
+ predictable seed.\n\
";
static int
not interpreter options. */
break;
}
- switch (c) {
- case 'E':
+ if (c == 'E') {
Py_IgnoreEnvironmentFlag++;
break;
- case 'R':
- Py_HashRandomizationFlag++;
- break;
}
}
- /* The variable is only tested for existence here; _PyRandom_Init will
- check its value further. */
- if (!Py_HashRandomizationFlag &&
- (p = Py_GETENV("PYTHONHASHSEED")) && *p != '\0')
- Py_HashRandomizationFlag = 1;
+ Py_HashRandomizationFlag = 1;
_PyRandom_Init();
PySys_ResetWarnOptions();
break;
case 'R':
- /* Already handled above */
+ /* Ignored */
break;
/* This space reserved for other options */
return;
_Py_HashSecret_Initialized = 1;
- /*
- By default, hash randomization is disabled, and only
- enabled if PYTHONHASHSEED is set to non-empty or if
- "-R" is provided at the command line:
- */
- if (!Py_HashRandomizationFlag) {
- /* Disable the randomized hash: */
- memset(secret, 0, secret_size);
- return;
- }
-
/*
Hash randomization is enabled. Generate a per-process secret,
using PYTHONHASHSEED if provided.
'-W', 'default', # Warnings set to 'default'
'-bb', # Warnings about bytes/bytearray
'-E', # Ignore environment variables
- '-R', # Randomize hashing
]
# Allow user-specified interpreter options to override our defaults.
args.extend(test.support.args_from_interpreter_flags())
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