ENH: Add `number.__class_getitem__`

4 files changed, 129 insertions, 0 deletions

diff --git a/numpy/__init__.pyi b/numpy/__init__.pyi
index f2c2329c6..58d5519de 100644
--- a/numpy/__init__.pyi
+++ b/numpy/__init__.pyi
@@ -3058,6 +3058,8 @@ class number(generic, Generic[_NBit1]):  # type: ignore
     def real(self: _ArraySelf) -> _ArraySelf: ...
     @property
     def imag(self: _ArraySelf) -> _ArraySelf: ...
+    if sys.version_info >= (3, 9):
+        def __class_getitem__(self, item: Any) -> GenericAlias: ...
     def __int__(self) -> int: ...
     def __float__(self) -> float: ...
     def __complex__(self) -> complex: ...
diff --git a/numpy/core/_add_newdocs.py b/numpy/core/_add_newdocs.py
index 2ac4d6cbc..6fc3b48e5 100644
--- a/numpy/core/_add_newdocs.py
+++ b/numpy/core/_add_newdocs.py
@@ -6500,6 +6500,37 @@ add_newdoc('numpy.core.numerictypes', 'generic',
 add_newdoc('numpy.core.numerictypes', 'generic',
            refer_to_array_attribute('view'))
 
+if sys.version_info >= (3, 9):
+    add_newdoc('numpy.core.numerictypes', 'number', ('__class_getitem__',
+        """
+        __class_getitem__(item, /)
+
+        Return a parametrized wrapper around the `~numpy.number` type.
+
+        .. versionadded:: 1.22
+
+        Returns
+        -------
+        alias : types.GenericAlias
+            A parametrized `~numpy.number` type.
+
+        Examples
+        --------
+        >>> from typing import Any
+        >>> import numpy as np
+
+        >>> np.signedinteger[Any]
+        numpy.signedinteger[typing.Any]
+
+        Note
+        ----
+        This method is only available for python 3.9 and later.
+
+        See Also
+        --------
+        :pep:`585` : Type hinting generics in standard collections.
+
+        """))
 
 ##############################################################################
 #
diff --git a/numpy/core/src/multiarray/scalartypes.c.src b/numpy/core/src/multiarray/scalartypes.c.src
index 4faa647ec..328581536 100644
--- a/numpy/core/src/multiarray/scalartypes.c.src
+++ b/numpy/core/src/multiarray/scalartypes.c.src
@@ -1806,6 +1806,21 @@ gentype_setflags(PyObject *NPY_UNUSED(self), PyObject *NPY_UNUSED(args),
 }
 
 /*
+ * Use for concrete np.number subclasses, making them act as if they
+ * were subtyped from e.g. np.signedinteger[object], thus lacking any
+ * free subscription parameters. Requires python >= 3.9.
+ */
+#ifdef Py_GENERICALIASOBJECT_H
+static PyObject *
+numbertype_class_getitem(PyObject *cls, PyObject *args)
+{
+    return PyErr_Format(PyExc_TypeError,
+                        "There are no type variables left in %s",
+                        ((PyTypeObject *)cls)->tp_name);
+}
+#endif
+
+/*
  * casting complex numbers (that don't inherit from Python complex)
  * to Python complex
  */
@@ -2188,6 +2203,16 @@ static PyGetSetDef inttype_getsets[] = {
     {NULL, NULL, NULL, NULL, NULL}
 };
 
+static PyMethodDef numbertype_methods[] = {
+    /* for typing; requires python >= 3.9 */
+    #ifdef Py_GENERICALIASOBJECT_H
+    {"__class_getitem__",
+        (PyCFunction)Py_GenericAlias,
+        METH_CLASS | METH_O, NULL},
+    #endif
+    {NULL, NULL, 0, NULL}  /* sentinel */
+};
+
 /**begin repeat
  * #name = cfloat,clongdouble#
  */
@@ -2195,6 +2220,12 @@ static PyMethodDef @name@type_methods[] = {
     {"__complex__",
         (PyCFunction)@name@_complex,
         METH_VARARGS | METH_KEYWORDS, NULL},
+    /* for typing; requires python >= 3.9 */
+    #ifdef Py_GENERICALIASOBJECT_H
+    {"__class_getitem__",
+        (PyCFunction)numbertype_class_getitem,
+        METH_CLASS | METH_O, NULL},
+    #endif
     {NULL, NULL, 0, NULL}
 };
 /**end repeat**/
@@ -2232,6 +2263,27 @@ static PyMethodDef @name@type_methods[] = {
     {"is_integer",
         (PyCFunction)@name@_is_integer,
         METH_NOARGS, NULL},
+    /* for typing; requires python >= 3.9 */
+    #ifdef Py_GENERICALIASOBJECT_H
+    {"__class_getitem__",
+        (PyCFunction)numbertype_class_getitem,
+        METH_CLASS | METH_O, NULL},
+    #endif
+    {NULL, NULL, 0, NULL}
+};
+/**end repeat**/
+
+/**begin repeat
+ * #name = byte, short, int, long, longlong, ubyte, ushort,
+ *         uint, ulong, ulonglong, timedelta, cdouble#
+ */
+static PyMethodDef @name@type_methods[] = {
+    /* for typing; requires python >= 3.9 */
+    #ifdef Py_GENERICALIASOBJECT_H
+    {"__class_getitem__",
+        (PyCFunction)numbertype_class_getitem,
+        METH_CLASS | METH_O, NULL},
+    #endif
     {NULL, NULL, 0, NULL}
 };
 /**end repeat**/
@@ -3951,6 +4003,8 @@ initialize_numeric_types(void)
 
     PyIntegerArrType_Type.tp_getset = inttype_getsets;
 
+    PyNumberArrType_Type.tp_methods = numbertype_methods;
+
     /**begin repeat
      * #NAME= Number, Integer, SignedInteger, UnsignedInteger, Inexact,
      *        Floating, ComplexFloating, Flexible, Character#
@@ -4016,6 +4070,17 @@ initialize_numeric_types(void)
 
     /**end repeat**/
 
+    /**begin repeat
+     * #name = byte, short, int, long, longlong, ubyte, ushort,
+     *         uint, ulong, ulonglong, timedelta, cdouble#
+     * #Name = Byte, Short, Int, Long, LongLong, UByte, UShort,
+     *         UInt, ULong, ULongLong, Timedelta, CDouble#
+     */
+
+    Py@Name@ArrType_Type.tp_methods = @name@type_methods;
+
+    /**end repeat**/
+
     /* We won't be inheriting from Python Int type. */
     PyIntArrType_Type.tp_hash = int_arrtype_hash;
 
diff --git a/numpy/core/tests/test_scalar_methods.py b/numpy/core/tests/test_scalar_methods.py
index 94b2dd3c9..0cdfe99b1 100644
--- a/numpy/core/tests/test_scalar_methods.py
+++ b/numpy/core/tests/test_scalar_methods.py
@@ -1,8 +1,11 @@
 """
 Test the scalar constructors, which also do type-coercion
 """
+import sys
 import fractions
 import platform
+import types
+from typing import Any, Type
 
 import pytest
 import numpy as np
@@ -128,3 +131,31 @@ class TestIsInteger:
             if value == 0:
                 continue
             assert not value.is_integer()
+
+
+@pytest.mark.skipif(sys.version_info < (3, 9), reason="Requires python 3.9")
+class TestClassGetItem:
+    @pytest.mark.parametrize("cls", [
+        np.number,
+        np.integer,
+        np.inexact,
+        np.unsignedinteger,
+        np.signedinteger,
+        np.floating,
+        np.complexfloating,
+    ])
+    def test_abc(self, cls: Type[np.number]) -> None:
+        alias = cls[Any]
+        assert isinstance(alias, types.GenericAlias)
+        assert alias.__origin__ is cls
+
+    @pytest.mark.parametrize("cls", [np.generic, np.flexible, np.character])
+    def test_abc_non_numeric(self, cls: Type[np.generic]) -> None:
+        with pytest.raises(TypeError):
+            cls[Any]
+
+    @pytest.mark.parametrize("code", np.typecodes["All"])
+    def test_concrete(self, code: str) -> None:
+        cls = np.dtype(code).type
+        with pytest.raises(TypeError):
+            cls[Any]