Query parameter

Learn query parameters with ?q=value.

Query parameter

If a function parameter is not in the path, FastAPI treats it as a query parameter. It is taken from the ?key=value part of the URL.

@app.get("/ara")
def ara(q: str = None):
    return {"q": q}
# /ara?q=kalem -> {"q": "kalem"}

If you give a default value, the parameter becomes optional; otherwise it is required. You can also give a type (int, str).

Required and optional

If you give a query parameter a default value (q: str = None) it becomes optional. Without one (q: str) it is required; if missing from the URL, 422 is returned.

# optional
@app.get("/ara")
def ara(q: str = None): ...
# required
@app.get("/ara")
def ara(q: str): ...

The default can be an empty string ("") or any other constant. This is useful for fields like search that can be left blank.

Exercise GET route with a query parameter

Start with from fastapi import FastAPI and from fastapi.testclient import TestClient. Define app = FastAPI(). With @app.get("/ara") define def ara(q: str = None): returning {"q": q}. Create a client with client = TestClient(app). Print the response body with print(client.get("/ara?q=kalem").text).

Show example solution

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get("/ara")
def ara(q: str = None):
    return {"q": q}

client = TestClient(app)
print(client.get("/ara?q=kalem").text)

Test yourself

Complete the function with a query parameter

@app.get("/ara")
def ara(____: str = None):
    return {"q": q}

Show answer

@app.get("/ara")
def ara(q: str = None):
    return {"q": q}

Which URL sends the query parameter q=kalem to /ara?
- /ara&q=kalem
- /ara/q/kalem
- /ara#q=kalem
- /ara?q=kalem
Show answer

/ara?q=kalem — The part starting with ? carries the query parameters.
Match the query parameter concepts

q: str = None

Is optional
q: str

Is required
?q=kalem

Is a URL query
def ara(q)

Is the function definition

Next lesson: Summary - FastAPI

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