Flask Request & Response – Handling Web Traffic - Learn PYTHON-BACKEND | Revochamp

Name: Flask Request & Response – Handling Web Traffic
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The Flask Request-Response Cycle

In Flask, every time a client visits a URL, Flask creates a Request object containing all the information sent by the browser. Once your function processes the logic, it must return a Response object. Understanding how to extract data from the request and format the response is the core of API development.

The Request Object

The request object is global within the context of a function call. It allows you to access URL parameters, form data, JSON payloads, and HTTP headers.

PythonRead-only

from flask import request

@app.route('/login', methods=['POST'])
def login():
    # Get JSON data from a POST request
    data = request.get_json()
    username = data.get('username')

    # Get Query Parameters from URL (?source=mobile)
    source = request.args.get('source')

    # Access Headers
    user_agent = request.headers.get('User-Agent')

    return f"Hello {username} from {source}"

from flask import request

@app.route('/login', methods=['POST'])
def login():
    # Get JSON data from a POST request
    data = request.get_json()
    username = data.get('username')

    # Get Query Parameters from URL (?source=mobile)
    source = request.args.get('source')

    # Access Headers
    user_agent = request.headers.get('User-Agent')

    return f"Hello {username} from {source}"

Creating a Response

By default, if you return a string or a dictionary, Flask automatically converts it into a response with a 200 OK status. However, for APIs, you often need to set custom status codes or headers using make_response or by returning a tuple.

PythonRead-only

from flask import make_response, jsonify

@app.route('/error')
def custom_error():
    # Option A: Simple Tuple (Body, Status, Headers)
    # return jsonify({"error": "Not Found"}), 404, {"X-Custom-Header": "Value"}

    # Option B: make_response for more control
    res = make_response(jsonify({"msg": "Auth Failed"}))
    res.status_code = 401
    res.headers['WWW-Authenticate'] = 'Basic realm="Login Required"'
    return res

from flask import make_response, jsonify

@app.route('/error')
def custom_error():
    # Option A: Simple Tuple (Body, Status, Headers)
    # return jsonify({"error": "Not Found"}), 404, {"X-Custom-Header": "Value"}

    # Option B: make_response for more control
    res = make_response(jsonify({"msg": "Auth Failed"}))
    res.status_code = 401
    res.headers['WWW-Authenticate'] = 'Basic realm="Login Required"'
    return res

Common Request Attributes

Attribute	Description	Example Use Case
request.args	URL Query parameters	Filtering search results (?q=python)
request.form	Form data from POST	Traditional HTML form submissions
request.json	Parsed JSON body	Modern REST API payloads
request.files	Uploaded files	Handling profile picture uploads
request.method	HTTP Method	Checking if request is GET or POST
request.cookies	Browser cookies	Reading session IDs

Visualizing the Flow

The flow starts at the client, passes through Flask's routing middleware, hits your view function, and returns through the response pipeline.

Test Your Knowledge

of 3

Which object is used to access the JSON data sent in a POST request?

request.json

response.json

flask.data

request.query

of 3

How do you return a 201 Created status code with a JSON response?

return jsonify(data).status(201)

return jsonify(data), 201

return 201, jsonify(data)

make_response(201, data)

of 3

Where are URL parameters like ?id=5 stored in the request object?

request.form

request.params

request.args

request.values

Frequently Asked Questions

What is the difference between request.args and request.form?

request.args contains parameters from the URL (query string). request.form contains data sent in the body of a POST request via an HTML form.

How do I set a cookie in the response?

You must use make_response() to get a response object, then call res.set_cookie('key', 'value') before returning it.

What happens if I don't return anything from a route?

Flask will raise a TypeError. A route must always return a string, a dictionary, a tuple, or a valid Response object.

from flask import request @app.route('/login', methods=['POST']) def login(): # Get JSON data from a POST request data = request.get_json() username = data.get('username') # Get Query Parameters from URL (?source=mobile) source = request.args.get('source') # Access Headers user_agent = request.headers.get('User-Agent') return f"Hello {username} from {source}"

from flask import make_response, jsonify @app.route('/error') def custom_error(): # Option A: Simple Tuple (Body, Status, Headers) # return jsonify({"error": "Not Found"}), 404, {"X-Custom-Header": "Value"} # Option B: make_response for more control res = make_response(jsonify({"msg": "Auth Failed"})) res.status_code = 401 res.headers['WWW-Authenticate'] = 'Basic realm="Login Required"' return res

Attribute

Description

Example Use Case

request.args

URL Query parameters

Filtering search results (?q=python)

request.form

Form data from POST

Traditional HTML form submissions

request.json

Parsed JSON body

Modern REST API payloads

request.files

Uploaded files

Handling profile picture uploads

request.method

HTTP Method

Checking if request is GET or POST

request.cookies

Browser cookies

Reading session IDs

Test Your Knowledge

of 3

Which object is used to access the JSON data sent in a POST request?

request.json

response.json

flask.data

request.query

of 3

How do you return a 201 Created status code with a JSON response?

return jsonify(data).status(201)

return jsonify(data), 201

return 201, jsonify(data)

make_response(201, data)

of 3

Where are URL parameters like ?id=5 stored in the request object?

request.form

request.params

request.args

request.values

Frequently Asked Questions

What is the difference between request.args and request.form?

request.args contains parameters from the URL (query string). request.form contains data sent in the body of a POST request via an HTML form.

How do I set a cookie in the response?

You must use make_response() to get a response object, then call res.set_cookie('key', 'value') before returning it.

What happens if I don't return anything from a route?

Flask will raise a TypeError. A route must always return a string, a dictionary, a tuple, or a valid Response object.

Flask Request & Response – Handling Web Traffic

Flask Request & Response – Handling Web Traffic

The Flask Request-Response Cycle

The Request Object

Creating a Response

Common Request Attributes

Visualizing the Flow

Test Your Knowledge

Frequently Asked Questions

Need help?

Flask Request & Response – Handling Web Traffic

Flask Request & Response – Handling Web Traffic

The Flask Request-Response Cycle

The Request Object

Creating a Response

Common Request Attributes

Visualizing the Flow

Test Your Knowledge

Frequently Asked Questions

Need help?