add on_setattr (#14)

* allow for custom repr function + bump version

* remove '_get_descriptors'

* make private classmethods functions

* improve readability

* add 'on_setattr'

tests/test_descriptor.py

@@ -179,3 +179,23 @@ class WithMetadata:
 def test_descriptor_metadata():
     """Test a descriptor with metadata."""
     assert WithMetadata.value.metadata["foo"] == "bar"
+
+
+class WithSetAttr:
+    """Modify the value before saving it."""
+
+    value = Descriptor(on_setattr=lambda self, value: value + 1)
+    value_frozen = Descriptor(frozen=True, on_setattr=lambda self, value: value + 1)
+
+
+def test_with_setattr():
+    """Test a descriptor with on_setattr."""
+    example = WithSetAttr()
+    example.value = 1
+    assert example.value == 2
+
+    example.value_frozen = 1
+    assert example.value_frozen == 2
+
+    with pytest.raises(TypeError):
+        example.value_frozen = 10

zninit/descriptor/__init__.py

@@ -57,6 +57,7 @@ def __init__(
         check_types: bool = False,
         metadata: dict = None,
         frozen: bool = False,
+        on_setattr: typing.Callable = None,
     ):  # pylint: disable=too-many-arguments
         """Define a Descriptor object.
 
@@ -81,6 +82,9 @@ def __init__(
             additional metadata for the descriptor.
         repr_func: Callable, default=repr
             A callable that will be used to compute the _repr_ of the descriptor.
+        on_setattr: Callable, default=None
+            A callable that is run whenever an attribute is set via 'class.myattr = value'
+             or 'setattr(class, "mattr", value)'.
         """
         self._default = default
         self._owner = owner
@@ -91,6 +95,7 @@ def __init__(
         self.metadata = metadata or {}
         self.frozen = frozen
         self.get_repr = repr_func
+        self.on_setattr = on_setattr
         self._frozen = weakref.WeakKeyDictionary()
         if check_types and ("typeguard" not in sys.modules):
             raise ImportError(
@@ -164,6 +169,8 @@ def __set__(self, instance, value):
         """Save value to instance.__dict__."""
         if self._frozen.get(instance, False):
             raise TypeError(f"Frozen attribute '{self.name}' can not be changed.")
+        if self.on_setattr is not None:
+            value = self.on_setattr(instance, value)
         if self.check_types:
             typeguard.check_type(
                 argname=self.name, value=value, expected_type=self.annotation