numpy ndarray hashability

Question

I have some problems understanding how numpy objects hashability is managed.

>>> import numpy as np
>>> class Vector(np.ndarray):
...     p         


        

Answer 1

This is not a clear answer, but here is some track to follow to understand this behavior.

I refer here to the numpy code of the 1.6.1 release.

According to numpy.ndarray object implementation (look at, numpy/core/src/multiarray/arrayobject.c), hash method is set to NULL.

NPY_NO_EXPORT PyTypeObject PyArray_Type = {
#if defined(NPY_PY3K)
    PyVarObject_HEAD_INIT(NULL, 0)
#else
    PyObject_HEAD_INIT(NULL)
    0,                                          /* ob_size */
#endif
    "numpy.ndarray",                            /* tp_name */
    sizeof(PyArrayObject),                      /* tp_basicsize */
    &array_as_mapping,                          /* tp_as_mapping */
    (hashfunc)0,                                /* tp_hash */

This tp_hash property seems to be overridden in numpy/core/src/multiarray/multiarraymodule.c. See DUAL_INHERIT, DUAL_INHERIT2 and initmultiarray function where tp_hash attribute is modified.

Ex: PyArrayDescr_Type.tp_hash = PyArray_DescrHash

According to hashdescr.c, hash is implemented as follow:

* How does this work ? The hash is computed from a list which contains all the
* information specific to a type. The hard work is to build the list
* (_array_descr_walk). The list is built as follows:
*      * If the dtype is builtin (no fields, no subarray), then the list
*      contains 6 items which uniquely define one dtype (_array_descr_builtin)
*      * If the dtype is a compound array, one walk on each field. For each
*      field, we append title, names, offset to the final list used for
*      hashing, and then append the list recursively built for each
*      corresponding dtype (_array_descr_walk_fields)
*      * If the dtype is a subarray, one adds the shape tuple to the list, and
*      then append the list recursively built for each corresponding type
*      (_array_descr_walk_subarray)