From 427c84f13f7719e6014a21bd1b81efdc02a046fb Mon Sep 17 00:00:00 2001
From: Sebastian Berg <sebastian@sipsolutions.net>
Date: Thu, 6 Feb 2020 06:54:05 -0800
Subject: bpo-39274: Ensure Fraction.__bool__() returns a bool (GH-18017)

Some numerator types used (specifically NumPy) decides to not
return a Python boolean for the "a != b" operation. Using the equivalent
call to bool() guarantees a bool return also for such types.
---
 Lib/test/test_fractions.py | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

(limited to 'Lib/test/test_fractions.py')

diff --git a/Lib/test/test_fractions.py b/Lib/test/test_fractions.py
index 7cf7899932..4649a34bcc 100644
--- a/Lib/test/test_fractions.py
+++ b/Lib/test/test_fractions.py
@@ -6,6 +6,7 @@ import math
 import numbers
 import operator
 import fractions
+import functools
 import sys
 import unittest
 import warnings
@@ -322,6 +323,42 @@ class FractionTest(unittest.TestCase):
 
         self.assertTypedEquals(0.1+0j, complex(F(1,10)))
 
+    def testBoolGuarateesBoolReturn(self):
+        # Ensure that __bool__ is used on numerator which guarantees a bool
+        # return.  See also bpo-39274.
+        @functools.total_ordering
+        class CustomValue:
+            denominator = 1
+
+            def __init__(self, value):
+                self.value = value
+
+            def __bool__(self):
+                return bool(self.value)
+
+            @property
+            def numerator(self):
+                # required to preserve `self` during instantiation
+                return self
+
+            def __eq__(self, other):
+                raise AssertionError("Avoid comparisons in Fraction.__bool__")
+
+            __lt__ = __eq__
+
+        # We did not implement all abstract methods, so register:
+        numbers.Rational.register(CustomValue)
+
+        numerator = CustomValue(1)
+        r = F(numerator)
+        # ensure the numerator was not lost during instantiation:
+        self.assertIs(r.numerator, numerator)
+        self.assertIs(bool(r), True)
+
+        numerator = CustomValue(0)
+        r = F(numerator)
+        self.assertIs(bool(r), False)
+
     def testRound(self):
         self.assertTypedEquals(F(-200), round(F(-150), -2))
         self.assertTypedEquals(F(-200), round(F(-250), -2))
-- 
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