Fix __eq__ protocol (#2197)

If it is not known how to compare objects, `__eq__` should return
NotImplemented instead of False. That way the right side object of the
comparison might possibly take over.

tests/test_basic.py

@@ -2,6 +2,7 @@
 import unittest
 
 from troposphere import (
+    AWSHelperFn,
     AWSObject,
     AWSProperty,
     Cidr,
@@ -25,6 +26,19 @@
 from troposphere.validators import positive_integer
 
 
+class TypeComparator:
+    """ Helper to test the __eq__ protocol """
+
+    def __init__(self, valid_types):
+        self.valid_types = valid_types
+
+    def __eq__(self, other):
+        return isinstance(other, self.valid_types)
+
+    def __ne__(self, other):
+        return not self == other
+
+
 def double(x):
     return positive_integer(x) * 2
 
@@ -77,8 +91,13 @@ def test___eq__(self):
             "title": "foobar",
             "Properties": {"callcorrect": True},
         }
+        assert FakeAWSObject("foobar", callcorrect=True) == TypeComparator(AWSObject)
+        assert TypeComparator(AWSObject) == FakeAWSObject("foobar", callcorrect=True)
+
         assert GenericHelperFn("foobar") == GenericHelperFn("foobar")
         assert GenericHelperFn({"foo": "bar"}) == {"foo": "bar"}
+        assert GenericHelperFn("foobar") == TypeComparator(AWSHelperFn)
+        assert TypeComparator(AWSHelperFn) == GenericHelperFn("foobar")
 
     def test___ne__(self):
         """Test __ne__."""
@@ -89,9 +108,14 @@ def test___ne__(self):
             "foobar", callcorrect=False
         )
         assert FakeAWSObject("foobar", callcorrect=True) != FakeAWSProperty("foobar")
+        assert FakeAWSObject("foobar", callcorrect=True) != TypeComparator(AWSHelperFn)
+        assert TypeComparator(AWSHelperFn) != FakeAWSObject("foobar", callcorrect=True)
+
         assert GenericHelperFn("foobar") != GenericHelperFn("bar")
         assert GenericHelperFn("foobar") != "foobar"
         assert GenericHelperFn("foobar") != FakeAWSProperty("foobar")
+        assert GenericHelperFn("foobar") != TypeComparator(AWSObject)
+        assert TypeComparator(AWSObject) != GenericHelperFn("foobar")
 
     def test_badproperty(self):
         with self.assertRaises(AttributeError):

tests/test_template.py

@@ -81,6 +81,19 @@ def test_max_mappings(self):
             template.add_mapping("mapping", {"n": "v"})
 
 
+class TypeComparator:
+    """ Helper to test the __eq__ protocol """
+
+    def __init__(self, valid_types):
+        self.valid_types = valid_types
+
+    def __eq__(self, other):
+        return isinstance(other, self.valid_types)
+
+    def __ne__(self, other):
+        return not self == other
+
+
 class TestEquality(unittest.TestCase):
     def test_eq(self):
         metadata = "foo"
@@ -98,6 +111,9 @@ def test_eq(self):
 
         self.assertEqual(t1, t2)
 
+        self.assertEqual(t1, TypeComparator(Template))
+        self.assertEqual(TypeComparator(Template), t1)
+
     def test_ne(self):
         t1 = Template(Description="foo1", Metadata="bar1")
         t1.add_resource(Bucket("Baz1"))
@@ -109,6 +125,9 @@ def test_ne(self):
 
         self.assertNotEqual(t1, t2)
 
+        self.assertNotEqual(t1, TypeComparator(Output))
+        self.assertNotEqual(TypeComparator(Output), t1)
+
     def test_hash(self):
         metadata = "foo"
         description = "bar"

troposphere/__init__.py

@@ -420,7 +420,7 @@ def __eq__(self, other: object) -> bool:
             return self.title == other.title and self.to_json() == other.to_json()
         if isinstance(other, dict):
             return {"title": self.title, **self.to_dict()} == other
-        return False
+        return NotImplemented
 
     def __ne__(self, other: object) -> bool:
         return not self == other
@@ -514,7 +514,7 @@ def __eq__(self, other: object) -> bool:
             return self.to_json() == other.to_json()
         if isinstance(other, (dict, list)):
             return self.to_dict() == other
-        return False
+        return NotImplemented
 
     def __hash__(self) -> int:
         return hash(self.to_json(indent=0))
@@ -986,10 +986,10 @@ def __eq__(self, other: object) -> bool:
         if isinstance(other, Template):
             return self.to_json() == other.to_json()
         else:
-            return False
+            return NotImplemented
 
     def __ne__(self, other: object) -> bool:
-        return not self.__eq__(other)
+        return not self == other
 
     def __hash__(self) -> int:
         return hash(self.to_json())