A basic Flask application requires creating a Flask instance and defining routes:

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello, World!'

if __name__ == '__main__':
    app.run(debug=True)

The Flask(__name__) creates the application instance, @app.route() decorator defines URL endpoints, and app.run() starts the development server.

The __name__ parameter helps Flask determine the root path of the application, which is used for:

• Resource location: Finding templates, static files, and other resources
• Import name: Used for extensions and debugging
• Module identification: Helps Flask understand the application's module structure

When run directly, __name__ equals '__main__', but when imported as a module, it contains the actual module name.

Specify HTTP methods using the methods parameter in the route decorator:

@app.route('/api/users', methods=['GET', 'POST'])
def handle_users():
    if request.method == 'POST':
        return create_user()
    return get_users()

@app.route('/api/users/<int:user_id>', methods=['PUT', 'DELETE'])
def handle_user(user_id):
    if request.method == 'PUT':
        return update_user(user_id)
    elif request.method == 'DELETE':
        return delete_user(user_id)

URL variables capture dynamic parts of URLs and pass them as function arguments:

@app.route('/user/<username>')
def show_user(username):
    return f'User: {username}'

@app.route('/post/<int:post_id>')
def show_post(post_id):
    return f'Post ID: {post_id}'

@app.route('/path/<path:subpath>')
def show_subpath(subpath):
    return f'Subpath: {subpath}'

Variable types include string (default), int, float, path, and uuid.

url_for() generates URLs for routes by endpoint name, providing URL reversal:

from flask import url_for

@app.route('/user/<username>')
def user_profile(username):
    return f'Profile for {username}'

@app.route('/')
def index():
    # Generate URL for user_profile endpoint
    profile_url = url_for('user_profile', username='john')
    return f'<a href="{profile_url}">John\'s Profile</a>'

Benefits include automatic URL updates when routes change and proper URL escaping.

Access query parameters using request.args:

from flask import request

@app.route('/search')
def search():
    query = request.args.get('q', '')
    page = request.args.get('page', 1, type=int)
    category = request.args.getlist('category')  # Multiple values
    
    return f'Query: {query}, Page: {page}, Categories: {category}'

Use get() for single values with defaults, getlist() for multiple values.

Flask provides the request object to access various types of request data:

from flask import request

@app.route('/form', methods=['POST'])
def handle_form():
    # Form data
    username = request.form['username']
    email = request.form.get('email', '')
    
    # JSON data
    if request.is_json:
        data = request.get_json()
    
    # Files
    if 'file' in request.files:
        file = request.files['file']
    
    # Headers
    auth_header = request.headers.get('Authorization')
    
    # Cookies
    session_id = request.cookies.get('session_id')
    
    return 'Data processed'

These access different types of request data:

• request.form: POST form data (application/x-www-form-urlencoded or multipart/form-data)
• request.args: URL query string parameters
• request.get_json(): JSON data from request body (application/json)

# URL: /api?page=1
# Body: {"name": "John"}
# Form data: username=admin

@app.route('/api', methods=['POST'])
def api():
    page = request.args.get('page')        # "1"
    name = request.get_json()['name']      # "John"
    username = request.form.get('username') # "admin"