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bpo-33919: Expose hash_seed and use_hash_seed in sys.hash_info #7867

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bpo-33919: Expose hash_seed and use_hash_seed in sys.hash_info #7867

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49 changes: 28 additions & 21 deletions Doc/library/sys.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -729,34 +729,41 @@ always available.
implementation. For more details about hashing of numeric types, see
:ref:`numeric-hash`.

+---------------------+--------------------------------------------------+
| attribute | explanation |
+=====================+==================================================+
| :const:`width` | width in bits used for hash values |
+---------------------+--------------------------------------------------+
| :const:`modulus` | prime modulus P used for numeric hash scheme |
+---------------------+--------------------------------------------------+
| :const:`inf` | hash value returned for a positive infinity |
+---------------------+--------------------------------------------------+
| :const:`nan` | hash value returned for a nan |
+---------------------+--------------------------------------------------+
| :const:`imag` | multiplier used for the imaginary part of a |
| | complex number |
+---------------------+--------------------------------------------------+
| :const:`algorithm` | name of the algorithm for hashing of str, bytes, |
| | and memoryview |
+---------------------+--------------------------------------------------+
| :const:`hash_bits` | internal output size of the hash algorithm |
+---------------------+--------------------------------------------------+
| :const:`seed_bits` | size of the seed key of the hash algorithm |
+---------------------+--------------------------------------------------+
+------------------------+-----------------------------------------------+
| attribute | explanation |
+========================+===============================================+
| :const:`width` | width in bits used for hash values |
+------------------------+-----------------------------------------------+
| :const:`modulus` | prime modulus P used for numeric hash scheme |
+------------------------+-----------------------------------------------+
| :const:`inf` | hash value returned for a positive infinity |
+------------------------+-----------------------------------------------+
| :const:`nan` | hash value returned for a nan |
+------------------------+-----------------------------------------------+
| :const:`imag` | multiplier used for the imaginary part of a |
| | complex number |
+------------------------+-----------------------------------------------+
| :const:`algorithm` | name of the algorithm for hashing of str, |
| | bytes, and memoryview |
+------------------------+-----------------------------------------------+
| :const:`hash_bits` | internal output size of the hash algorithm |
+------------------------+-----------------------------------------------+
| :const:`seed_bits` | size of the seed key of the hash algorithm |
+------------------------+-----------------------------------------------+
| :const:`use_hash_seed` | Whether to use :envvar:`PYTHONHASHSEED` |
+------------------------+-----------------------------------------------+
| :const:`hash_seed` | The value from :envvar:`PYTHONHASHSEED` |

@vstinner vstinner Jun 23, 2018

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At the C level, we need two fields, since it's not possible to distinguish "value set" to "value unset". But at the Python level, we can use None when the hash seed is unset.

@vstinner vstinner Jun 23, 2018

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It seems like the seed is always in the range [0; 4294967295], _Py_ReadHashSeed:

        if (*endptr != '\0'
            || seed > 4294967295UL
            || (errno == ERANGE && seed == ULONG_MAX))
        {
            return -1;
        }

It may be worth to document the value range.

+------------------------+-----------------------------------------------+


.. versionadded:: 3.2

.. versionchanged:: 3.4
Added *algorithm*, *hash_bits* and *seed_bits*

.. versionchanged:: 3.8
Added *use_hash_seed* and *hash_seed*


.. data:: hexversion

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4 changes: 3 additions & 1 deletion Lib/test/test_sys.py
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Expand Up @@ -427,7 +427,7 @@ def test_attributes(self):
self.assertEqual(type(sys.int_info.sizeof_digit), int)
self.assertIsInstance(sys.hexversion, int)

self.assertEqual(len(sys.hash_info), 9)
self.assertEqual(len(sys.hash_info), 11)
self.assertLess(sys.hash_info.modulus, 2**sys.hash_info.width)
# sys.hash_info.modulus should be a prime; we do a quick
# probable primality test (doesn't exclude the possibility of
Expand Down Expand Up @@ -458,6 +458,8 @@ def test_attributes(self):
self.assertEqual(algo, 0)
self.assertGreaterEqual(sys.hash_info.cutoff, 0)
self.assertLess(sys.hash_info.cutoff, 8)
self.assertIsInstance(sys.hash_info.use_hash_seed, bool)
self.assertIsInstance(sys.hash_info.hash_seed, int)

self.assertIsInstance(sys.maxsize, int)
self.assertIsInstance(sys.maxunicode, int)
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2018-06-22-17-28-08.bpo-33919.axMvlh.rst
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@@ -0,0 +1,3 @@
Attributes ``use_hash_seed`` and ``hash_seed`` have been added to
``sys.hash_info``. These reflect the setting of the
:envvar:`PYTHONHASHSEED` environment variable.
10 changes: 9 additions & 1 deletion Python/sysmodule.c
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Expand Up @@ -932,25 +932,29 @@ static PyStructSequence_Field hash_info_fields[] = {
{"hash_bits", "internal output size of hash algorithm"},
{"seed_bits", "seed size of hash algorithm"},
{"cutoff", "small string optimization cutoff"},
{"use_hash_seed", "whether the hash seed should be used or not"},
{"hash_seed", "the value of the hash seed"},
{NULL, NULL}
};

static PyStructSequence_Desc hash_info_desc = {
"sys.hash_info",
hash_info_doc,
hash_info_fields,
9,
11,
};

static PyObject *
get_hash_info(void)
{
_PyCoreConfig *core_config = &_PyGILState_GetInterpreterStateUnsafe()->core_config;
PyObject *hash_info;
int field = 0;
PyHash_FuncDef *hashfunc;
hash_info = PyStructSequence_New(&Hash_InfoType);
if (hash_info == NULL)
return NULL;

hashfunc = PyHash_GetFuncDef();
PyStructSequence_SET_ITEM(hash_info, field++,
PyLong_FromLong(8*sizeof(Py_hash_t)));
Expand All @@ -970,6 +974,10 @@ get_hash_info(void)
PyLong_FromLong(hashfunc->seed_bits));
PyStructSequence_SET_ITEM(hash_info, field++,
PyLong_FromLong(Py_HASH_CUTOFF));
PyStructSequence_SET_ITEM(hash_info, field++,
PyBool_FromLong(core_config->use_hash_seed));
PyStructSequence_SET_ITEM(hash_info, field++,
PyLong_FromLong(core_config->hash_seed));
if (PyErr_Occurred()) {
Py_CLEAR(hash_info);
return NULL;
Expand Down