Middleware in Python

A middleware wraps every request: timing, headers, logging.

import time

from fastapi import FastAPI, Request
from fastapi.middleware.cors import CORSMiddleware
from fastapi.testclient import TestClient

app = FastAPI()
app.add_middleware(CORSMiddleware, allow_origins=["https://typestar.io"])


@app.middleware("http")
async def add_timing(request: Request, call_next):
    started = time.perf_counter()
    response = await call_next(request)
    elapsed = (time.perf_counter() - started) * 1000
    response.headers["X-Elapsed-Ms"] = f"{elapsed:.1f}"
    return response


@app.get("/ping")
def ping():
    return {"pong": True}


client = TestClient(app)
response = client.get("/ping")
print(response.json(), "X-Elapsed-Ms" in response.headers)

How it works

1. The function receives the request and a call_next.
2. Whatever it returns is the response the client gets.
3. CORS is a middleware the framework ships.

Keywords and builtins used here

• add_timing
• async
• await
• def
• ping
• print
• return

The run, in numbers

Lines

27

Characters to type

664

Tokens

159

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 87 seconds.

Type this snippet

Step 3 of 3 in Dependencies & async, step 12 of 19 in Web services & data access.

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Middleware in other languages

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