set previous values

codeforlife/models/signals/pre_save.py

@@ -25,6 +25,24 @@ def adding(instance: _.AnyModel):
     return instance._state.adding
 
 
+def _generate_get_previous_value(
+    instance: _.AnyModel,
+) -> t.Callable[[str], t.Any]:
+    if adding(instance):
+        # pylint: disable-next=unused-argument
+        def get_previous_value(field: str):
+            return None
+
+    else:
+        objects = instance.__class__.objects  # type: ignore[attr-defined]
+        previous_instance = objects.get(pk=instance.pk)
+
+        def get_previous_value(field: str):
+            return getattr(previous_instance, field)
+
+    return get_previous_value
+
+
 def check_previous_values(
     instance: _.AnyModel,
     predicates: t.Dict[str, t.Callable[[t.Any, t.Any], bool]],
@@ -42,24 +60,31 @@ def check_previous_values(
     """
     # pylint: enable=line-too-long
 
-    if adding(instance):
-        # pylint: disable-next=unused-argument
-        def get_previous_value(field: str):
-            return None
-
-    else:
-        objects = instance.__class__.objects  # type: ignore[attr-defined]
-        previous_instance = objects.get(pk=instance.pk)
-
-        def get_previous_value(field: str):
-            return getattr(previous_instance, field)
+    get_previous_value = _generate_get_previous_value(instance)
 
     return all(
         predicate(get_previous_value(field), getattr(instance, field))
         for field, predicate in predicates.items()
     )
 
 
+def set_previous_values(instance: _.AnyModel, fields: t.Set[str]):
+    # pylint: disable=line-too-long
+    """Set the previous value of the specified fields. All fields are set on the
+    instance with the naming convention: "previous_{field}".
+
+    Args:
+        instance: The current instance.
+        fields: The fields to get the previous value for.
+    """
+    # pylint: enable=line-too-long
+
+    get_previous_value = _generate_get_previous_value(instance)
+
+    for field in fields:
+        setattr(instance, f"previous_{field}", get_previous_value(field))
+
+
 def previous_values_are_unequal(instance: _.AnyModel, fields: t.Set[str]):
     # pylint: disable=line-too-long
     """Check if all the previous values are not equal to the current values. If