Sync fastapi docs from b5ca1324 on 2025-12-07

docs_src/query_params_str_validations/tutorial015_an.py

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+import random
+from typing import Union
+
+from fastapi import FastAPI
+from pydantic import AfterValidator
+from typing_extensions import Annotated
+
+app = FastAPI()
+
+data = {
+    "isbn-9781529046137": "The Hitchhiker's Guide to the Galaxy",
+    "imdb-tt0371724": "The Hitchhiker's Guide to the Galaxy",
+    "isbn-9781439512982": "Isaac Asimov: The Complete Stories, Vol. 2",
+}
+
+
+def check_valid_id(id: str):
+    if not id.startswith(("isbn-", "imdb-")):
+        raise ValueError('Invalid ID format, it must start with "isbn-" or "imdb-"')
+    return id
+
+
+@app.get("/items/")
+async def read_items(
+    id: Annotated[Union[str, None], AfterValidator(check_valid_id)] = None,
+):
+    if id:
+        item = data.get(id)
+    else:
+        id, item = random.choice(list(data.items()))
+    return {"id": id, "name": item}