Merge #877

877: UnitContainer: hash collision does not guarantee equality r=hgrecco a=crusaderky

Closes #876

Co-authored-by: Guido Imperiale <guido.imperiale@amphorainc.com>

pint/testsuite/test_issues.py

@@ -11,7 +11,7 @@
 from pint.unit import UnitsContainer
 from pint.util import ParserHelper
 
-from pint.compat import np, long_type
+from pint.compat import np
 from pint.errors import UndefinedUnitError, DimensionalityError
 from pint.testsuite import QuantityTestCase, helpers
 
@@ -699,3 +699,21 @@ def test_issue856b(self):
         ureg2.define('test123 = 456 kg')
         assert ureg1('1 test123').to('kg').magnitude == 123
         assert ureg2('1 test123').to('kg').magnitude == 456
+
+    def test_issue876(self):
+        # Same hash must not imply equality.
+
+        # As an implementation detail of CPython, hash(-1) == hash(-2).
+        # This test is useless in potential alternative Python implementations where
+        # hash(-1) != hash(-2); one would need to find hash collisions specific for each
+        # implementation
+
+        a = UnitsContainer({"[mass]": -1})
+        b = UnitsContainer({"[mass]": -2})
+        c = UnitsContainer({"[mass]": -3})
+
+        # Guarantee working on alternative Python implementations
+        assert (hash(-1) == hash(-2)) == (hash(a) == hash(b))
+        assert (hash(-1) == hash(-3)) == (hash(a) == hash(c))
+        assert a != b
+        assert a != c

pint/util.py

@@ -327,9 +327,16 @@ def __setstate__(self, state):
 
     def __eq__(self, other):
         if isinstance(other, UnitsContainer):
-            # Not the same as hash(self); see ParserHelper.__hash__ and __eq__
-            return UnitsContainer.__hash__(self) == UnitsContainer.__hash__(other)
-        if isinstance(other, string_types):
+            # UnitsContainer.__hash__(self) is not the same as hash(self); see
+            # ParserHelper.__hash__ and __eq__.
+            # Different hashes guarantee that the actual contents are different, but
+            # identical hashes give no guarantee of equality.
+            # e.g. in CPython, hash(-1) == hash(-2)
+            if UnitsContainer.__hash__(self) != UnitsContainer.__hash__(other):
+                return False
+            other = other._d
+
+        elif isinstance(other, string_types):
             other = ParserHelper.from_string(other)
             other = other._d
 
@@ -504,9 +511,11 @@ def __setstate__(self, state):
         self._d, self._hash, self.scale = state
 
     def __eq__(self, other):
-        if isinstance(other, self.__class__):
-            return self.scale == other.scale and\
+        if isinstance(other, ParserHelper):
+            return (
+                self.scale == other.scale and
                 super(ParserHelper, self).__eq__(other)
+            )
         elif isinstance(other, string_types):
             return self == ParserHelper.from_string(other)
         elif isinstance(other, Number):