get previous value

codeforlife/models/signals/post_save.py

@@ -11,6 +11,8 @@
 from . import general as _
 from .pre_save import PREVIOUS_VALUE_KEY
 
+FieldValue = t.TypeVar("FieldValue")
+
 
 def has_previous_values(instance: _.AnyModel, fields: t.Dict[str, t.Type]):
     # pylint: disable=line-too-long
@@ -36,3 +38,27 @@ def has_previous_values(instance: _.AnyModel, fields: t.Dict[str, t.Type]):
             return False
 
     return True
+
+
+def get_previous_value(
+    instance: _.AnyModel, field: str, cls: t.Type[FieldValue]
+):
+    # pylint: disable=line-too-long
+    """Get a previous value from the instance and assert the value is of the
+    expected type.
+
+    Args:
+        instance: The current instance.
+        field: The field to get the previous value for.
+        cls: The expected type of the value.
+
+    Returns:
+        The previous value of the field.
+    """
+    # pylint: enable=line-too-long
+
+    previous_value = getattr(instance, PREVIOUS_VALUE_KEY.format(field=field))
+
+    assert isinstance(previous_value, cls)
+
+    return previous_value