bpo-24234: implement complex.__complex__

Lib/test/test_complex.py

@@ -499,6 +499,18 @@ def __complex__(self):
             self.assertEqual(complex(complex1(1j)), 2j)
         self.assertRaises(TypeError, complex, complex2(1j))
 
+    def test___complex__(self):
+        z = 3 + 4j
+        self.assertEqual(z.__complex__(), z)
+        self.assertEqual(type(z.__complex__()), complex)
+
+        class complex_subclass(complex):
+            pass
+
+        z = complex_subclass(3 + 4j)
+        self.assertEqual(z.__complex__(), 3 + 4j)
+        self.assertEqual(type(z.__complex__()), complex)
+
     @support.requires_IEEE_754
     def test_constructor_special_numbers(self):
         class complex2(complex):

Lib/test/test_doctest.py

@@ -668,7 +668,7 @@ def non_Python_modules(): r"""
 
     >>> import builtins
     >>> tests = doctest.DocTestFinder().find(builtins)
-    >>> 816 < len(tests) < 836 # approximate number of objects with docstrings
+    >>> 820 < len(tests) < 840 # approximate number of objects with docstrings
     True
     >>> real_tests = [t for t in tests if len(t.examples) > 0]
     >>> len(real_tests) # objects that actually have doctests

Lib/test/test_typing.py

@@ -1533,11 +1533,11 @@ def test_supports_float(self):
 
     def test_supports_complex(self):
 
-        # Note: complex itself doesn't have __complex__.
         class C:
             def __complex__(self):
                 return 0j
 
+        self.assertIsSubclass(complex, typing.SupportsComplex)
         self.assertIsSubclass(C, typing.SupportsComplex)
         self.assertNotIsSubclass(str, typing.SupportsComplex)
 

Misc/NEWS.d/next/Core and Builtins/2021-08-22-12-28-50.bpo-24234.n3oTdx.rst

@@ -0,0 +1,3 @@
+Implement the :meth:`__complex__` special method on the :class:`complex` type,
+so a complex number ``z`` passes an ``isinstance(z, typing.SupportsComplex)``
+check.

Objects/complexobject.c

@@ -693,8 +693,29 @@ complex___format___impl(PyComplexObject *self, PyObject *format_spec)
     return _PyUnicodeWriter_Finish(&writer);
 }
 
+/*[clinic input]
+complex.__complex__
+
+Convert this value to exact type complex.
+[clinic start generated code]*/
+
+static PyObject *
+complex___complex___impl(PyComplexObject *self)
+/*[clinic end generated code: output=e6b35ba3d275dc9c input=3589ada9d27db854]*/
+{
+    if (PyComplex_CheckExact(self)) {
+        Py_INCREF(self);
+        return (PyObject *)self;
+    }
+    else {
+        return PyComplex_FromCComplex(self->cval);
+    }
+}
+
+
 static PyMethodDef complex_methods[] = {
     COMPLEX_CONJUGATE_METHODDEF
+    COMPLEX___COMPLEX___METHODDEF
     COMPLEX___GETNEWARGS___METHODDEF
     COMPLEX___FORMAT___METHODDEF
     {NULL,              NULL}           /* sentinel */