Add field-default-null

Author: kalekseev

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ### Unreleased
 
+- Add checks:
+    - field-default-null
+
 ### 0.5.0
 
 - Fix ignore_checks

README.md

@@ -74,6 +74,11 @@ EXTRA_CHECKS = {
 - **field-boolean-null** - prefer using `BooleanField(null=True)` instead of `NullBooleanField`.
 - **field-null** - don't pass `null=False` to model fields (this is django default).
 - **field-foreign-key-index** - ForeignKey fields must specify `db_index` explicitly (to apply to unique together only: `when: unique_together`).
+- **field-default-null** - If field nullable (`null=True`), then
+    `default=None` argument is redundant and should be removed.
+    **WARNING** Be aware that setting is database dependent,
+    eg. Oracle interprets empty strings as nulls as a result
+    django uses empty string instead of null as default.
 
 ### DRF Serializers
 

src/extra_checks/check_id.py

@@ -15,5 +15,6 @@ class CheckId(str, enum.Enum):
     X056 = "field-boolean-null"
     X057 = "field-null"
     X058 = "field-foreign-key-db-index"
+    X059 = "field-default-null"
     X301 = "drf-model-serializer-extra-kwargs"
     X302 = "drf-model-serializer-meta-attribute"

src/extra_checks/checks/model_field_checks.py

@@ -231,3 +231,18 @@ def apply(
                         hint="Specify `db_index` field argument.",
                         obj=field,
                     )
+
+
+@registry.register(django.core.checks.Tags.models)
+class CheckFieldDefaultNull(CheckModelField):
+    Id = CheckId.X059
+
+    def apply(
+        self, field: models.fields.Field, field_ast: FieldAST, **kwargs: Any
+    ) -> Iterator[django.core.checks.CheckMessage]:
+        if field.null and field.default is None and "default" in field_ast.kwargs:
+            yield self.message(
+                "Argument `default=None` is redundant if `null=True` is set. (see docs about exceptions).",
+                hint="Remove `default=None` from field arguments.",
+                obj=field,
+            )

tests/example/models.py

@@ -92,6 +92,11 @@ class ModelFieldNullFalse(models.Model):
     null_fail = models.NullBooleanField()
 
 
+class ModelFieldNullDefault(models.Model):
+    myfield = models.IntegerField(default=None)
+    myfield_fail = models.IntegerField(null=True, default=None)
+
+
 class ModelFieldForeignKeyIndex(models.Model):
     article = models.ForeignKey(Article, on_delete=models.CASCADE, related_name="+")
     author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name="+")

tests/test_model_field_checks.py

@@ -228,3 +228,16 @@ def test_field_ignore_types(test_case):
     )
     assert len(messages) == 1
     assert {m.obj.name for m in messages} == {"file_fail"}
+
+
+def test_field_null_default_null(test_case):
+    messages = (
+        test_case.settings(
+            {"checks": [model_field_checks.CheckFieldDefaultNull.Id.value]}
+        )
+        .models(models.ModelFieldNullDefault)
+        .check(model_field_checks.CheckFieldDefaultNull)
+        .run()
+    )
+    assert len(messages) == 1
+    assert messages[0].obj.name == "myfield_fail"