Accept a list of strings as value for JSON Schema "type"

According to the JSON Schema specification [1], the "type" keyword may
be an array of strings (with unique items). Previously, the Schema
definition did not allow this and would thus prevent users from using
custom JSON Schema definitions in their field such as `Annotated[...,
WithJsonSchema({"type": ["string", "null"]})` as an alternative to the
`anyOf` which is usually generated (a ValidationError was raised when
generated the OpenAPI definition).

We now accept this "type" variant, as illustrated in modified
test_custom_schema_fields.py.

[1]: https://json-schema.org/draft/2020-12/json-schema-validation#name-type

fastapi/openapi/models.py

@@ -157,7 +157,7 @@ class Schema(BaseModelWithConfig):
     unevaluatedProperties: Optional["SchemaOrBool"] = None
     # Ref: JSON Schema Validation 2020-12: https://json-schema.org/draft/2020-12/json-schema-validation.html#name-a-vocabulary-for-structural
     # A Vocabulary for Structural Validation
-    type: Optional[str] = None
+    type: Optional[Union[str, Set[str]]] = None
     enum: Optional[List[Any]] = None
     const: Optional[Any] = None
     multipleOf: Optional[float] = Field(default=None, gt=0)

tests/test_custom_schema_fields.py

@@ -1,7 +1,14 @@
+from typing import Optional
+
+from dirty_equals import IsList
 from fastapi import FastAPI
 from fastapi._compat import PYDANTIC_V2
 from fastapi.testclient import TestClient
 from pydantic import BaseModel
+from typing_extensions import Annotated
+
+if PYDANTIC_V2:
+    from pydantic.json_schema import WithJsonSchema
 
 app = FastAPI()
 
@@ -10,12 +17,17 @@ class Item(BaseModel):
     name: str
 
     if PYDANTIC_V2:
+        description: Annotated[
+            Optional[str], WithJsonSchema({"type": ["string", "null"]})
+        ] = None
+
         model_config = {
             "json_schema_extra": {
                 "x-something-internal": {"level": 4},
             }
         }
     else:
+        description: Optional[str] = None  # type: ignore[no-redef]
 
         class Config:
             schema_extra = {
@@ -42,7 +54,18 @@ item_schema = {
         "name": {
             "title": "Name",
             "type": "string",
-        }
+        },
+        "description": (
+            {
+                "title": "Description",
+                "type": IsList("string", "null", check_order=False),
+            }
+            if PYDANTIC_V2
+            else {
+                "title": "Description",
+                "type": "string",
+            }
+        ),
     },
 }
 
@@ -57,4 +80,4 @@ def test_response():
     # For coverage
     response = client.get("/foo")
     assert response.status_code == 200, response.text
-    assert response.json() == {"name": "Foo item"}
+    assert response.json() == {"name": "Foo item", "description": None}