Merge pull request #554 from wearebasti/bugfix-decimal-places-522

fix: inherit decimal places from arguments in arithmetic operations #522

djmoney/money.py

@@ -26,27 +26,40 @@ def __init__(self, *args, **kwargs):
         self.decimal_places = kwargs.pop("decimal_places", DECIMAL_PLACES)
         super().__init__(*args, **kwargs)
 
+    def _fix_decimal_places(self, *args):
+        """ Make sure to user 'biggest' number of decimal places of all given money instances """
+        candidates = (getattr(candidate, "decimal_places", 0) for candidate in args)
+        return max([self.decimal_places, *candidates])
+
     def __add__(self, other):
         if isinstance(other, F):
             return other.__radd__(self)
         other = maybe_convert(other, self.currency)
-        return super().__add__(other)
+        result = super().__add__(other)
+        result.decimal_places = self._fix_decimal_places(other)
+        return result
 
     def __sub__(self, other):
         if isinstance(other, F):
             return other.__rsub__(self)
         other = maybe_convert(other, self.currency)
-        return super().__sub__(other)
+        result = super().__sub__(other)
+        result.decimal_places = self._fix_decimal_places(other)
+        return result
 
     def __mul__(self, other):
         if isinstance(other, F):
             return other.__rmul__(self)
-        return super().__mul__(other)
+        result = super().__mul__(other)
+        result.decimal_places = self._fix_decimal_places(other)
+        return result
 
     def __truediv__(self, other):
         if isinstance(other, F):
             return other.__rtruediv__(self)
-        return super().__truediv__(other)
+        result = super().__truediv__(other)
+        result.decimal_places = self._fix_decimal_places(other)
+        return result
 
     @property
     def is_localized(self):
@@ -70,6 +83,14 @@ def __round__(self, n=None):
         amount = round(self.amount, n)
         return self.__class__(amount, self.currency)
 
+    # DefaultMoney sets those synonym functions
+    # we overwrite the 'targets' so the wrong synonyms are called
+    # Example: we overwrite __add__; __radd__ calls __add__ on DefaultMoney...
+    __radd__ = __add__
+    __rsub__ = __sub__
+    __rmul__ = __mul__
+    __rtruediv__ = __truediv__
+
 
 def get_current_locale():
     # get_language can return None starting from Django 1.8

docs/changes.rst

@@ -3,6 +3,15 @@
 Changelog
 =========
 
+`1.2`_ - TBA
+------------
+
+Fixed
+~~~~~
+
+- Resulting Money object from arithmetics (add / sub / ...) inherits maximum decimal_places from arguments `#522`_ (`wearebasti`_)
+
+
 `1.1`_ - 2020-04-06
 -------------------
 
@@ -786,6 +795,7 @@ Added
 .. _#418: https://github.com/django-money/django-money/issues/418
 .. _#411: https://github.com/django-money/django-money/issues/411
 .. _#519: https://github.com/django-money/django-money/issues/519
+.. _#522: https://github.com/django-money/django-money/issues/522
 
 .. _77cc33: https://github.com/77cc33
 .. _AlexRiina: https://github.com/AlexRiina
@@ -859,3 +869,4 @@ Added
 .. _humrochagf: https://github.com/humrochagf
 .. _washeck: https://github.com/washeck
 .. _fara: https://github.com/fara
+.. _wearebasti: https://github.com/wearebasti

tests/test_money.py

@@ -42,3 +42,45 @@ def test_round():
 def test_configurable_decimal_number():
     # Override default configuration per instance
     assert str(Money("10.543", "USD", decimal_places=3)) == "$10.543"
+
+
+def test_add_decimal_places():
+    one = Money("1.0000", "USD", decimal_places=4)
+    two = Money("2.000000005", "USD", decimal_places=10)
+
+    result = one + two
+    assert result.decimal_places == 10
+
+
+def test_add_decimal_places_zero():
+    two = Money("2.005", "USD", decimal_places=3)
+
+    result = two + 0
+    assert result.decimal_places == 3
+
+
+def test_mul_decimal_places():
+    """ Test __mul__ and __rmul__ """
+    two = Money("1.0000", "USD", decimal_places=4)
+
+    result = 2 * two
+    assert result.decimal_places == 4
+
+    result = two * 2
+    assert result.decimal_places == 4
+
+
+def test_fix_decimal_places():
+    one = Money(1, "USD", decimal_places=7)
+    assert one._fix_decimal_places(Money(2, "USD", decimal_places=3)) == 7
+    assert one._fix_decimal_places(Money(2, "USD", decimal_places=30)) == 30
+
+
+def test_fix_decimal_places_none():
+    one = Money(1, "USD", decimal_places=7)
+    assert one._fix_decimal_places(None) == 7
+
+
+def test_fix_decimal_places_multiple():
+    one = Money(1, "USD", decimal_places=7)
+    assert one._fix_decimal_places(None, Money(3, "USD", decimal_places=8)) == 8