🔊 [#4927] Add system check for missing allow_blank on non-required ch…

…arfields

src/openforms/utils/checks.py

@@ -1,9 +1,12 @@
+import inspect
 import os
+from typing import Type
 
 from django.conf import settings
 from django.core.checks import Error, Warning, register
 from django.forms import ModelForm
 
+from rest_framework.serializers import CharField, Serializer, empty
 from treebeard.forms import MoveNodeForm
 
 
@@ -81,3 +84,58 @@ def check_missing_init_files(app_configs, **kwargs):
         )
 
     return errors
+
+
+def get_field_line_number(
+    serializer_class: Type[Serializer], field_name: str
+) -> int | None:
+    # Get source lines and the starting line number of the serializer class
+    source_lines, start_line_number = inspect.getsourcelines(serializer_class)
+
+    # Find the line containing the field definition
+    for i, line in enumerate(source_lines):
+        if line.strip().startswith(f"{field_name} ="):
+            return start_line_number + i  # Compute the exact line number
+
+    return None  # Return None if field definition is not found
+
+
+@register
+def check_serializer_non_required_charfield_allow_blank_true(app_configs, **kwargs):
+    """
+    https://github.com/open-formulieren/open-forms/issues/4927
+
+    Check for serializers.CharFields that have required=False, but not `allow_blank=True`
+    to avoid errors occurring when empty strings are passed from the frontend.
+    """
+    errors = []
+    serializers = get_subclasses(Serializer)
+    for serializer in serializers:
+        # TODO not sure if this issue can also occur for fields defined in Meta.fields?
+        for field_name, field in serializer._declared_fields.items():
+            if not isinstance(field, CharField) or field.read_only:
+                continue
+
+            if (
+                not field.required
+                and field.default in ("", None, empty)
+                and not field.allow_blank
+            ):
+                file_path = inspect.getfile(serializer)
+                line_number = get_field_line_number(serializer, field_name)
+
+                # No line number indicates that the field is inherited from another class
+                if not file_path or not line_number:
+                    continue
+
+                errors.append(
+                    Warning(
+                        (
+                            f"{serializer.__module__}.{serializer.__qualname__}.{field_name} does not have `allow_blank=True`\n"
+                            f"{file_path}:{line_number}"
+                        ),
+                        hint="Consider setting `allow_blank=True` to allow passing of empty strings",
+                        id="utils.W002",
+                    )
+                )
+    return errors