Request & Response
Reading request data and building responses in Flask
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Explanation
Flask exposes the current request through the and lets you build responses with helper functions like .
Reading request data:
python from flask import Flask, request, jsonify app = Flask(__name__) @app.route("/users", methods=["POST"]) def create_user(): # JSON body data = request.get_json() # dict or None name = data.get("name") email = data.get("email") # Query parameters: /users?page=2&limit=10 page = request.args.get("page", 1, type=int) limit = request.args.get("limit", 10, type=int) # Form data (Content-Type: multipart/form-data) name = request.form.get("name") # Headers token = request.headers.get("Authorization") # Route parameters are passed as function arguments return jsonify({"ok": True}), 201
Building responses:
python from flask import jsonify, make_response # jsonify — sets Content-Type: application/json return jsonify({"id": 1, "name": "Alice"}), 200 # Custom headers response = make_response(jsonify({"ok": True}), 201) response.headers["X-Request-ID"] = "abc123" return response # Empty 204 return "", 204
Validation pattern:
python @app.route("/users", methods=["POST"]) def create_user(): data = request.get_json(silent=True) if not data: return jsonify({"error": "JSON body required"}), 400 if not data.get("email"): return jsonify({"error": "email is required"}), 422 # ... process return jsonify({"id": new_id}), 201
Examples
Paginated list endpoint
Read query params with type coercion and build a paginated response
from flask import Flask, request, jsonify
app = Flask(__name__)
USERS = [{"id": i, "name": f"User {i}"} for i in range(1, 101)]
@app.route("/users")
def list_users():
page = request.args.get("page", 1, type=int)
limit = request.args.get("limit", 10, type=int)
if page < 1 or limit < 1 or limit > 100:
return jsonify({"error": "Invalid pagination params"}), 400
start = (page - 1) * limit
end = start + limit
items = USERS[start:end]
return jsonify({
"data": items,
"page": page,
"limit": limit,
"total": len(USERS),
})File upload handling
File uploads via request.files — always secure_filename to prevent path traversal
import os
from flask import Flask, request, jsonify
from werkzeug.utils import secure_filename
app = Flask(__name__)
app.config["UPLOAD_FOLDER"] = "/tmp/uploads"
ALLOWED = {"png", "jpg", "pdf"}
@app.route("/upload", methods=["POST"])
def upload():
if "file" not in request.files:
return jsonify({"error": "No file"}), 400
f = request.files["file"]
if not f.filename:
return jsonify({"error": "Empty filename"}), 400
ext = f.filename.rsplit(".", 1)[-1].lower()
if ext not in ALLOWED:
return jsonify({"error": f"Extension .{ext} not allowed"}), 415
filename = secure_filename(f.filename)
f.save(os.path.join(app.config["UPLOAD_FOLDER"], filename))
return jsonify({"filename": filename}), 201How well did you understand this?
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