Repository: django-money/django-money

Issue:
`Money.decimal_places` are not always the same after arithmetic operations
`djmoney.money.Money` uses some methods from `moneyed.Money`. In these methods (e.g. `__neg__`) new instances are created with only `amount` and `currency`. For this reason, our `django-money` specific attributes are not copied.

Example:

```python
amount = -Money(1, "USD", decimal_places=7)
amount.decimal_places  # 2
```

Comments:

Patch:
diff --git a/djmoney/money.py b/djmoney/money.py
index b744c66..f109fdc 100644
--- a/djmoney/money.py
+++ b/djmoney/money.py
@@ -39,17 +39,27 @@ class Money(DefaultMoney):
         """ Set number of digits being displayed - `None` resets to `DECIMAL_PLACES_DISPLAY` setting """
         self._decimal_places_display = value
 
-    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 _copy_attributes(self, source, target):
+        """Copy attributes to the new `Money` instance.
+
+        This class stores extra bits of information about string formatting that the parent class doesn't have.
+        The problem is that the parent class creates new instances of `Money` without in some of its methods and
+        it does so without knowing about `django-money`-level attributes.
+        For this reason, when this class uses some methods of the parent class that have this behavior, the resulting
+        instances lose those attribute values.
+
+        When it comes to what number of decimal places to choose, we take the maximum number.
+        """
+        for attribute_name in ("decimal_places", "decimal_places_display"):
+            value = max([getattr(candidate, attribute_name, 0) for candidate in (self, source)])
+            setattr(target, attribute_name, value)
 
     def __add__(self, other):
         if isinstance(other, F):
             return other.__radd__(self)
         other = maybe_convert(other, self.currency)
         result = super().__add__(other)
-        result.decimal_places = self._fix_decimal_places(other)
+        self._copy_attributes(other, result)
         return result
 
     def __sub__(self, other):
@@ -57,14 +67,14 @@ class Money(DefaultMoney):
             return other.__rsub__(self)
         other = maybe_convert(other, self.currency)
         result = super().__sub__(other)
-        result.decimal_places = self._fix_decimal_places(other)
+        self._copy_attributes(other, result)
         return result
 
     def __mul__(self, other):
         if isinstance(other, F):
             return other.__rmul__(self)
         result = super().__mul__(other)
-        result.decimal_places = self._fix_decimal_places(other)
+        self._copy_attributes(other, result)
         return result
 
     def __truediv__(self, other):
@@ -72,11 +82,11 @@ class Money(DefaultMoney):
             return other.__rtruediv__(self)
         result = super().__truediv__(other)
         if isinstance(result, self.__class__):
-            result.decimal_places = self._fix_decimal_places(other)
+            self._copy_attributes(other, result)
         return result
 
     def __rtruediv__(self, other):
-        # Backported from py-moneyd, non released bug-fix
+        # Backported from py-moneyed, non released bug-fix
         # https://github.com/py-moneyed/py-moneyed/blob/c518745dd9d7902781409daec1a05699799474dd/moneyed/classes.py#L217-L218
         raise TypeError("Cannot divide non-Money by a Money instance.")
 
@@ -100,7 +110,34 @@ class Money(DefaultMoney):
 
     def __round__(self, n=None):
         amount = round(self.amount, n)
-        return self.__class__(amount, self.currency)
+        new = self.__class__(amount, self.currency)
+        self._copy_attributes(self, new)
+        return new
+
+    def round(self, ndigits=0):
+        new = super().round(ndigits)
+        self._copy_attributes(self, new)
+        return new
+
+    def __pos__(self):
+        new = super().__pos__()
+        self._copy_attributes(self, new)
+        return new
+
+    def __neg__(self):
+        new = super().__neg__()
+        self._copy_attributes(self, new)
+        return new
+
+    def __abs__(self):
+        new = super().__abs__()
+        self._copy_attributes(self, new)
+        return new
+
+    def __rmod__(self, other):
+        new = super().__rmod__(other)
+        self._copy_attributes(self, new)
+        return new
 
     # DefaultMoney sets those synonym functions
     # we overwrite the 'targets' so the wrong synonyms are called
diff --git a/docs/changes.rst b/docs/changes.rst
index 8c0b272..f526c12 100644
--- a/docs/changes.rst
+++ b/docs/changes.rst
@@ -16,6 +16,7 @@ Changelog
 
 - Pin ``pymoneyed<1.0`` as it changed the ``repr`` output of the ``Money`` class.
 - Subtracting ``Money`` from ``moneyed.Money``. Regression, introduced in ``1.2``. `#593`_
+- Missing the right ``Money.decimal_places`` and ``Money.decimal_places_display`` values after some arithmetic operations. `#595`_
 
 `1.2.2`_ - 2020-12-29
 ---------------------
@@ -695,6 +696,7 @@ wrapping with ``money_manager``.
 .. _0.3: https://github.com/django-money/django-money/compare/0.2...0.3
 .. _0.2: https://github.com/django-money/django-money/compare/0.2...a6d90348085332a393abb40b86b5dd9505489b04
 
+.. _#595: https://github.com/django-money/django-money/issues/595
 .. _#593: https://github.com/django-money/django-money/issues/593
 .. _#586: https://github.com/django-money/django-money/issues/586
 .. _#585: https://github.com/django-money/django-money/pull/585


Test patch:
diff --git a/tests/test_money.py b/tests/test_money.py
index bb7ea79..e736854 100644
--- a/tests/test_money.py
+++ b/tests/test_money.py
@@ -80,31 +80,31 @@ def test_add_decimal_places_zero():
     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
+@pytest.mark.parametrize("decimal_places", (1, 4))
+@pytest.mark.parametrize(
+    "operation",
+    (
+        lambda a, d: a * 2,
+        lambda a, d: 2 * a,
+        lambda a, d: a / 5,
+        lambda a, d: a - Money("2", "USD", decimal_places=d, decimal_places_display=d),
+        lambda a, d: Money("2", "USD", decimal_places=d, decimal_places_display=d) - a,
+        lambda a, d: a + Money("2", "USD", decimal_places=d, decimal_places_display=d),
+        lambda a, d: Money("2", "USD", decimal_places=d, decimal_places_display=d) + a,
+        lambda a, d: -a,
+        lambda a, d: +a,
+        lambda a, d: abs(a),
+        lambda a, d: 5 % a,
+        lambda a, d: round(a),
+        lambda a, d: a.round(),
+    ),
+)
+def test_keep_decimal_places(operation, decimal_places):
+    # Arithmetic operations should keep the `decimal_places` value
+    amount = Money("1.0000", "USD", decimal_places=decimal_places, decimal_places_display=decimal_places)
+    new = operation(amount, decimal_places)
+    assert new.decimal_places == decimal_places
+    assert new.decimal_places_display == decimal_places
 
 
 def test_decimal_places_display_overwrite():