Accept function values for IntoMoney

Fixes #7.

src/budge/money.py

@@ -1,18 +1,29 @@
+from collections.abc import Callable
 from decimal import Decimal
-from typing import Any, Union
+from typing import Any
 
-from stockholm import Money, MoneyType
+from stockholm import Money
 from stockholm.money import MoneyModel
 
-IntoMoneyType = Union[MoneyType, MoneyModel[Any], Decimal, int, float, str, object]
+type IntoMoneyType = MoneyModel[Money] | Decimal | int | float | str
+
+type IntoMoneyCallback = Callable[[Any], IntoMoneyType]
 
 
 class IntoMoney:
     def __set_name__(self, owner: Any, name: str):
         self._name = "_" + name
 
-    def __get__(self, instance, owner=None) -> Money:
-        return getattr(instance, self._name)
+    def __get__(self, instance: Any, owner=None) -> Money:
+        attr: Money | IntoMoneyCallback = getattr(instance, self._name)
+
+        if isinstance(attr, Money):
+            return attr
+
+        return Money(attr(instance))
+
+    def __set__(self, instance, value: IntoMoneyType | IntoMoneyCallback):
+        if not callable(value):
+            value = Money(value)
 
-    def __set__(self, instance, value: IntoMoneyType):
-        setattr(instance, self._name, Money(value))
+        setattr(instance, self._name, value)

tests/budge/test_money.py

@@ -23,11 +23,25 @@ def test_into_money_from_str():
     assert obj.amount == Money(100)
 
 
-def test_into_money_from_none():
-    with raises(ConversionError, match="Missing input values for monetary amount"):
-        IntoMoneyTest(None)
-
-
 def test_into_money_from_empty_string():
     with raises(ConversionError, match="Missing input values for monetary amount"):
         IntoMoneyTest("")
+
+
+def test_into_money_with_callable():
+    def get_amount(instance: IntoMoneyTest):
+        assert instance is obj
+        return Money(100)
+
+    obj = IntoMoneyTest(get_amount)
+    assert isinstance(obj.amount, Money)
+    assert obj.amount == Money(100)
+
+
+def test_into_money_with_wrong_callable_type():
+    def wrong_function(instance: IntoMoneyTest):
+        return "can't convert this to Money"
+
+    obj = IntoMoneyTest(wrong_function)
+    with raises(ConversionError, match="Input value cannot be used as monetary amount"):
+        obj.amount