feat: model signal helpers

codeforlife/models/__init__.py

@@ -0,0 +1,17 @@
+import typing as t
+
+from django.db.models import Model as _Model
+
+
+class Model(_Model):
+    """A base class for all Django models.
+
+    Args:
+        _Model (django.db.models.Model): Django's model class.
+    """
+
+    id: int
+    pk: int
+
+
+AnyModel = t.TypeVar("AnyModel", bound=Model)

codeforlife/models/signals.py

@@ -1,3 +1,69 @@
 import typing as t
 
+from . import AnyModel
+
 UpdateFields = t.Optional[t.FrozenSet[str]]
+
+
+def check_post_save(
+    update_fields: UpdateFields,
+    expected_update_fields: UpdateFields,
+):
+    """https://docs.djangoproject.com/en/3.2/ref/signals/#post-save
+
+    Check if a model was updated with the expected fields.
+
+    Args:
+        update_fields: The fields that were updated.
+        expected_update_fields: A subset of the fields that were expected to be
+            updated. If no fields were expected to be updated, set to None.
+
+    Returns:
+        If the model instance was updated as expected.
+    """
+
+    if expected_update_fields is None:
+        return update_fields is None
+    elif update_fields is None:
+        return False
+
+    return all(
+        update_field in update_fields for update_field in expected_update_fields
+    )
+
+
+def check_pre_save(
+    update_fields: UpdateFields = None,
+    expected_update_fields: UpdateFields = None,
+    instance: t.Optional[AnyModel] = None,
+    created: bool = False,
+    created_only: bool = False,
+):
+    """https://docs.djangoproject.com/en/3.2/ref/signals/#pre-save
+
+    Check if a model was created or updated with the expected fields.
+
+    Args:
+        update_fields: The fields that were updated.
+        expected_update_fields: A subset of the fields that were expected to be
+            updated. If no fields were expected to be updated, set to None.
+        instance: Any model instance.
+        created: Check if the model was created.
+        created_only: Only check if the model was created.
+
+    Raises:
+        ValueError: If arg 'created' is True and arg 'instance' is None.
+
+    Returns:
+        If the model instance was created or updated as expected.
+    """
+
+    if created:
+        if instance is None:
+            raise ValueError("Arg 'instance' cannot be None.")
+        if created_only:
+            return instance.pk is None
+        if instance.pk is None:
+            return True
+
+    return check_post_save(update_fields, expected_update_fields)