Improve Json field detection and simplify tests

fastapi/dependencies/utils.py

@@ -716,15 +716,20 @@ def _validate_value_with_model_field(
         return v_, []
 
 
-def _is_pydantic_json_field(field: ModelField) -> bool:
+def _is_json_field(field: ModelField) -> bool:
-    return any(item.__class__.__name__ == 'Json' for item in field.field_info.metadata)
+    # Pydantic v1 FieldInfo doesn't have metadata attribute
+    metadata = getattr(field.field_info, "metadata", [])
+    return any(
+        type(item).__name__ == "Json" and type(item).__module__ == "pydantic.types"
+        for item in metadata
+    )
 
 
 def _get_multidict_value(
     field: ModelField, values: Mapping[str, Any], alias: Union[str, None] = None
 ) -> Any:
     alias = alias or get_validation_alias(field)
-    if _is_pydantic_json_field(field):
+    if _is_json_field(field):
         value = values.get(alias, None)
     elif is_sequence_field(field) and isinstance(values, (ImmutableMultiDict, Headers)):
         value = values.getlist(alias)

tests/test_form_json_type.py

@@ -1,325 +1,44 @@
-"""
-Test cases for Form() with Pydantic Json[T] type.
-Regression tests for issue #10997.
-https://github.com/fastapi/fastapi/issues/10997
-
-Before the fix: Json[list[str]] with Form() would wrap values in extra list
-After the fix: _is_pydantic_json_field() ensures correct single value extraction
-"""
-
 import json
-from pathlib import Path
 from typing import Annotated
 
-from dirty_equals import IsDict
+from fastapi import FastAPI, Form
-from fastapi import FastAPI, File, Form, UploadFile
 from fastapi.testclient import TestClient
 from pydantic import BaseModel, Json
 
 app = FastAPI()
 
 
-# ============================================================================
+class JsonListModel(BaseModel):
-# Endpoints
+    json_list: Json[list[str]]
-# ============================================================================
-
-
-@app.post("/form-json-list")
-def form_json_list(items: Annotated[Json[list[str]], Form()]) -> list[str]:
-    """Primary bug case - Json[list[str]] with Form()."""
-    return items
-
-
-@app.post("/form-json-dict")
-def form_json_dict(data: Annotated[Json[dict[str, str]], Form()]) -> dict[str, str]:
-    """Json[dict] parsing."""
-    return data
-
-
-@app.post("/form-json-optional")
-def form_json_optional(
-    items: Annotated[Json[list[str]] | None, Form()] = None
-) -> dict:
-    """Optional Json field."""
-    return {"items": items, "received": items is not None}
-
-
-@app.post("/form-json-with-regular-fields")
-def form_json_with_regular_fields(
-    username: Annotated[str, Form()],
-    tags: Annotated[Json[list[str]], Form()],
-    age: Annotated[int, Form()],
-) -> dict:
-    """Json field mixed with regular Form fields."""
-    return {"username": username, "tags": tags, "age": age}
 
 
-class FormWithJsonModel(BaseModel):
+@app.post("/form-str")
-    """Pydantic model with Json field for Form()."""
+def form_str(json_list: Annotated[str, Form()]) -> list[str]:
+    model = JsonListModel(json_list=json_list) # type: ignore[arg-type]
+    return model.json_list
 
-    username: str
-    tags: Json[list[str]]
-
-
-@app.post("/form-json-model")
-def form_json_model(form: Annotated[FormWithJsonModel, Form()]) -> FormWithJsonModel:
-    """Json field in Pydantic model with Form()."""
-    return form
-
-
-@app.post("/form-json-with-file")
-def form_json_with_file(
-    tags: Annotated[Json[list[str]], Form()],
-    file: UploadFile = File(...),
-) -> dict:
-    """Json field with File() parameter."""
-    return {"tags": tags, "filename": file.filename}
-
-
-@app.post("/form-regular-list")
-def form_regular_list(items: Annotated[list[str], Form()]) -> list[str]:
-    """Regular list Form field (without Json wrapper) - must still work."""
-    return items
 
+@app.post("/form-json-list")
+def form_json_list(json_list: Annotated[Json[list[str]], Form()]) -> list[str]:
+    return json_list
 
-# ============================================================================
-# Test Client
-# ============================================================================
 
 client = TestClient(app)
 
 
-# ============================================================================
+def test_form_str():
-# Core Functionality Tests
-# ============================================================================
-
-
-def test_form_json_list_str():
-    """Test Form() + Json[list[str]] - primary bug case for issue #10997.
-
-    Before the fix, this would incorrectly wrap the JSON string in a list:
-        Input: '["abc", "def"]'
-        Bug:   [['["abc", "def"]']]  (wrapped in extra list)
-        Fix:   ["abc", "def"]        (correctly parsed)
-    """
     response = client.post(
-        "/form-json-list",
+        "/form-str",
-        data={"items": json.dumps(["abc", "def"])},
+        data={"json_list": json.dumps(["abc", "def"])},
     )
     assert response.status_code == 200, response.text
-    result = response.json()
+    assert response.json() == ["abc", "def"]
-
-    # Verify correct parsing
-    assert result == ["abc", "def"]
-
-    # Explicitly verify NOT the buggy wrapped version
-    assert result != [['["abc", "def"]']]
 
 
-def test_form_json_dict():
+def test_form_json_list():
-    """Test Form() + Json[dict] - dictionary parsing."""
-    test_data = {"name": "John", "city": "NYC"}
-    response = client.post(
-        "/form-json-dict",
-        data={"data": json.dumps(test_data)},
-    )
-    assert response.status_code == 200, response.text
-    assert response.json() == test_data
-
-
-# ============================================================================
-# Edge Cases Tests
-# ============================================================================
-
-
-def test_form_json_optional_with_none():
-    """Test optional Json field with no value sent."""
-    response = client.post("/form-json-optional", data={})
-    assert response.status_code == 200, response.text
-    result = response.json()
-    assert result["items"] is None
-    assert result["received"] is False
-
-
-def test_form_json_optional_with_value():
-    """Test optional Json field with value provided."""
-    response = client.post(
-        "/form-json-optional",
-        data={"items": json.dumps(["test"])},
-    )
-    assert response.status_code == 200, response.text
-    result = response.json()
-    assert result["items"] == ["test"]
-    assert result["received"] is True
-
-
-def test_form_json_empty_values():
-    """Test Json fields with empty array and object."""
-    # Empty list
     response = client.post(
         "/form-json-list",
-        data={"items": json.dumps([])},
+        data={"json_list": json.dumps(["abc", "def"])},
     )
     assert response.status_code == 200, response.text
-    assert response.json() == []
+    assert response.json() == ["abc", "def"]
-
-    # Empty dict
-    response = client.post(
-        "/form-json-dict",
-        data={"data": json.dumps({})},
-    )
-    assert response.status_code == 200, response.text
-    assert response.json() == {}
-
-
-def test_form_json_with_regular_fields():
-    """Test Json field mixed with regular Form fields."""
-    response = client.post(
-        "/form-json-with-regular-fields",
-        data={
-            "username": "alice",
-            "tags": json.dumps(["python", "fastapi"]),
-            "age": "30",
-        },
-    )
-    assert response.status_code == 200, response.text
-    assert response.json() == {
-        "username": "alice",
-        "tags": ["python", "fastapi"],
-        "age": 30,
-    }
-
-
-# ============================================================================
-# Error Handling Tests
-# ============================================================================
-
-
-def test_form_json_missing_required():
-    """Test missing required Json field returns validation error."""
-    response = client.post("/form-json-list", data={})
-    assert response.status_code == 422, response.text
-    error_detail = response.json()
-    # Note: input can be None or {} depending on form data processing
-    assert error_detail == IsDict(
-        {
-            "detail": [
-                {
-                    "type": "missing",
-                    "loc": ["body", "items"],
-                    "msg": "Field required",
-                    "input": None,
-                }
-            ]
-        }
-    ) | IsDict(
-        {
-            "detail": [
-                {
-                    "type": "missing",
-                    "loc": ["body", "items"],
-                    "msg": "Field required",
-                    "input": {},
-                }
-            ]
-        }
-    ) | IsDict(
-        # Pydantic v1 compatibility
-        {
-            "detail": [
-                {
-                    "loc": ["body", "items"],
-                    "msg": "field required",
-                    "type": "value_error.missing",
-                }
-            ]
-        }
-    )
-
-
-# ============================================================================
-# Integration Tests
-# ============================================================================
-
-
-def test_form_json_with_pydantic_model():
-    """Test Json field in Pydantic model with Form()."""
-    response = client.post(
-        "/form-json-model",
-        data={
-            "username": "alice",
-            "tags": json.dumps(["python", "fastapi"]),
-        },
-    )
-    assert response.status_code == 200, response.text
-    assert response.json() == {
-        "username": "alice",
-        "tags": ["python", "fastapi"],
-    }
-
-
-def test_form_json_with_file_upload(tmp_path: Path):
-    """Test Json field with File() parameter in multipart form."""
-    temp_file = tmp_path / "test.txt"
-    temp_file.write_text("test content")
-
-    response = client.post(
-        "/form-json-with-file",
-        data={"tags": json.dumps(["important", "urgent"])},
-        files={"file": ("test.txt", temp_file.read_bytes())},
-    )
-    assert response.status_code == 200, response.text
-    result = response.json()
-    assert result["tags"] == ["important", "urgent"]
-    assert result["filename"] == "test.txt"
-
-
-def test_form_json_doesnt_break_regular_sequences():
-    """Test that regular list Form fields still work correctly.
-
-    This is critical: the fix for Json[T] must not break existing
-    Form list behavior. Regular list fields should still use getlist()
-    for multiple form values.
-    """
-    response = client.post(
-        "/form-regular-list",
-        data={"items": ["x", "y", "z"]},
-    )
-    assert response.status_code == 200, response.text
-    assert response.json() == ["x", "y", "z"]
-
-
-# ============================================================================
-# OpenAPI Schema Test
-# ============================================================================
-
-
-def test_openapi_schema_json_field():
-    """Test that OpenAPI schema correctly represents Json fields as string type.
-
-    In OpenAPI, Json[T] form fields should be represented as string type
-    (not the inner type like array or object), because the form data contains
-    a JSON string that will be parsed by Pydantic, not the parsed structure.
-    """
-    response = client.get("/openapi.json")
-    assert response.status_code == 200, response.text
-
-    schema = response.json()
-    paths = schema["paths"]
-
-    # Check /form-json-list endpoint
-    endpoint_schema = paths["/form-json-list"]["post"]
-    assert "application/x-www-form-urlencoded" in endpoint_schema["requestBody"]["content"]
-
-    # Get body schema
-    body_schema_ref = endpoint_schema["requestBody"]["content"][
-        "application/x-www-form-urlencoded"
-    ]["schema"]["$ref"]
-    schema_name = body_schema_ref.split("/")[-1]
-    body_schema = schema["components"]["schemas"][schema_name]
-
-    # Verify Json[list[str]] is represented as string in OpenAPI
-    items_field = body_schema["properties"]["items"]
-    assert items_field["type"] == "string", (
-        f"Json[list[str]] should be 'string' type in OpenAPI, got {items_field}"
-    )