bpo-44547: Make Fractions objects instances of typing.SupportsInt (GH-27851)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/test_fractions.py

@@ -8,6 +8,7 @@ import operator
 import fractions
 import functools
 import sys
+import typing
 import unittest
 from copy import copy, deepcopy
 import pickle
@@ -385,6 +386,47 @@ class FractionTest(unittest.TestCase):
 
         self.assertTypedEquals(0.1+0j, complex(F(1,10)))
 
+    def testSupportsInt(self):
+        # See bpo-44547.
+        f = F(3, 2)
+        self.assertIsInstance(f, typing.SupportsInt)
+        self.assertEqual(int(f), 1)
+        self.assertEqual(type(int(f)), int)
+
+    def testIntGuaranteesIntReturn(self):
+        # Check that int(some_fraction) gives a result of exact type `int`
+        # even if the fraction is using some other Integral type for its
+        # numerator and denominator.
+
+        class CustomInt(int):
+            """
+            Subclass of int with just enough machinery to convince the Fraction
+            constructor to produce something with CustomInt numerator and
+            denominator.
+            """
+
+            @property
+            def numerator(self):
+                return self
+
+            @property
+            def denominator(self):
+                return CustomInt(1)
+
+            def __mul__(self, other):
+                return CustomInt(int(self) * int(other))
+
+            def __floordiv__(self, other):
+                return CustomInt(int(self) // int(other))
+
+        f = F(CustomInt(13), CustomInt(5))
+
+        self.assertIsInstance(f.numerator, CustomInt)
+        self.assertIsInstance(f.denominator, CustomInt)
+        self.assertIsInstance(f, typing.SupportsInt)
+        self.assertEqual(int(f), 2)
+        self.assertEqual(type(int(f)), int)
+
     def testBoolGuarateesBoolReturn(self):
         # Ensure that __bool__ is used on numerator which guarantees a bool
         # return.  See also bpo-39274.