gh-125420: implement Sequence.index API on memoryview objects

Doc/library/stdtypes.rst

@@ -4149,6 +4149,15 @@ copying.
       .. versionchanged:: 3.5
          The source format is no longer restricted when casting to a byte view.
 
+   .. method:: index(value, start=0, stop=sys.maxsize, /)
+
+      Return the index of the first occurrence of *value* (at or after
+      index *start* and before index *stop*).
+
+      Raises a :exc:`ValueError` if *value* cannot be found.
+
+      .. versionadded:: next
+
    There are also several readonly attributes available:
 
    .. attribute:: obj

Lib/test/test_memoryview.py

@@ -15,6 +15,7 @@
 import pickle
 import struct
 
+from itertools import product
 from test.support import import_helper
 
 
@@ -58,6 +59,31 @@ def test_getitem(self):
         for tp in self._types:
             self.check_getitem_with_type(tp)
 
+    def test_index(self):
+        for tp in self._types:
+            b = tp(self._source)
+            m = self._view(b)  # may be a sub-view
+            l = m.tolist()
+            k = 2 * len(self._source)
+
+            for chi in self._source:
+                if chi in l:
+                    self.assertEqual(m.index(chi), l.index(chi))
+                else:
+                    self.assertRaises(ValueError, m.index, chi)
+
+                for start, stop in product(range(-k, k), range(-k, k)):
+                    index = -1
+                    try:
+                        index = l.index(chi, start, stop)
+                    except ValueError:
+                        pass
+
+                    if index == -1:
+                        self.assertRaises(ValueError, m.index, chi, start, stop)
+                    else:
+                        self.assertEqual(m.index(chi, start, stop), index)
+
     def test_iter(self):
         for tp in self._types:
             b = tp(self._source)

Misc/NEWS.d/next/Core_and_Builtins/2024-10-14-13-28-16.gh-issue-125420.hNKixM.rst

@@ -0,0 +1,2 @@
+Add :meth:`memoryview.index` to :class:`memoryview` objects. Patch by
+Bénédikt Tran.

Objects/memoryobject.c

@@ -2748,6 +2748,92 @@ static PySequenceMethods memory_as_sequence = {
 };
 
 
+/**************************************************************************/
+/*                             Lookup                                     */
+/**************************************************************************/
+
+/*[clinic input]
+memoryview.index
+
+    value: object
+    start: slice_index(accept={int}) = 0
+    stop: slice_index(accept={int}, c_default="PY_SSIZE_T_MAX") = sys.maxsize
+    /
+
+Return the index of the first occurrence of a value.
+
+Raises ValueError if the value is not present.
+[clinic start generated code]*/
+
+static PyObject *
+memoryview_index_impl(PyMemoryViewObject *self, PyObject *value,
+                      Py_ssize_t start, Py_ssize_t stop)
+/*[clinic end generated code: output=e0185e3819e549df input=0697a0165bf90b5a]*/
+{
+    const Py_buffer *view = &self->view;
+    CHECK_RELEASED(self);
+
+    if (view->ndim == 0) {
+        PyErr_SetString(PyExc_TypeError, "invalid lookup on 0-dim memory");
+        return NULL;
+    }
+
+    if (view->ndim == 1) {
+        Py_ssize_t n = view->shape[0];
+
+        if (start < 0) {
+            start = Py_MAX(start + n, 0);
+        }
+
+        if (stop < 0) {
+            stop = Py_MAX(stop + n, 0);
+        }
+
+        stop = Py_MIN(stop, n);
+        assert(stop >= 0);
+        assert(stop <= n);
+
+        start = Py_MIN(start, stop);
+        assert(0 <= start);
+        assert(start <= stop);
+
+        PyObject *obj = _PyObject_CAST(self);
+        for (Py_ssize_t index = start; index < stop; index++) {
+            // Note: while memoryviews can be mutated during iterations
+            // when calling the == operator, their shape cannot. As such,
+            // it is safe to assume that the index remains valid for the
+            // entire loop.
+            assert(index < n);
+
+            PyObject *item = memory_item(obj, index);
+            if (item == NULL) {
+                return NULL;
+            }
+            if (item == value) {
+                Py_DECREF(item);
+                return PyLong_FromSsize_t(index);
+            }
+            int contained = PyObject_RichCompareBool(item, value, Py_EQ);
+            Py_DECREF(item);
+            if (contained > 0) {  // more likely than 'contained < 0'
+                return PyLong_FromSsize_t(index);
+            }
+            else if (contained < 0) {
+                return NULL;
+            }
+        }
+
+        PyErr_SetString(PyExc_ValueError, "memoryview.index(x): x not found");
+        return NULL;
+    }
+
+    PyErr_SetString(PyExc_NotImplementedError,
+                    "multi-dimensional lookup is not implemented");
+    return NULL;
+
+}
+
+
 /**************************************************************************/
 /*                             Comparisons                                */
 /**************************************************************************/
@@ -3284,6 +3370,7 @@ static PyMethodDef memory_methods[] = {
     MEMORYVIEW_CAST_METHODDEF
     MEMORYVIEW_TOREADONLY_METHODDEF
     MEMORYVIEW__FROM_FLAGS_METHODDEF
+    MEMORYVIEW_INDEX_METHODDEF
     {"__enter__",   memory_enter, METH_NOARGS, NULL},
     {"__exit__",    memory_exit, METH_VARARGS, memory_exit_doc},
     {"__class_getitem__", Py_GenericAlias, METH_O|METH_CLASS, PyDoc_STR("See PEP 585")},