Flask Routing Basics

# CHAPTER 5

Flask Routing Basics #

1. Introduction #

2. Learning Objectives #

• Define multiple static routes within a single application.

• Create dynamic routes that capture variables from the URL.

• Use type converters (<int:id>) to validate URL parameters.

• Restrict routes to specific HTTP methods using methods=['GET', 'POST'].

3. Beginner-Friendly Explanation #

• Static Routing: A letter arrives addressed to "The Mayor." You know exactly which bin to put it in because there is only one Mayor. (@app.route('/about')).

• Dynamic Routing: A letter arrives addressed to "Citizen #402". You don't have a specific bin for every single citizen. Instead, you have a general "Citizens" conveyor belt. You pull the number "402" off the envelope and hand it to the delivery driver, saying, "Take this to whoever is number 402." (@app.route('/user/<id>')).

Dynamic routing allows one single Python function to handle millions of different URLs automatically.

4. Step 1: Multiple Static Routes #

from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return "Homepage"

@app.route('/about')
def about():
    return "About Our Company"

@app.route('/contact')
def contact():
    return "Contact Us: support@example.com"

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

*You can define as many static routes as you want. Flask reads the URL and triggers the matching function.*

5. Step 2: Dynamic Routes (Capturing Variables) #

# The <username> part acts as a wildcard
@app.route(&#039;/profile/<username>')
def profile(username):
    # The URL data is passed into the function!
    return f"<h1>Welcome to your profile, {username}!</h1>"

*Test it! Visit http://127.0.0.1:5000/profile/alice. The browser prints "Welcome to your profile, alice!" Visit /profile/bob, and it prints "bob". One function handles infinite URLs.*

6. Step 3: URL Type Converters #

# Force the 'post_id' to be an integer
@app.route(&#039;/post/<int:post_id>')
def show_post(post_id):
    return f"Viewing Blog Post #{post_id}"

*If a user visits /post/5, it works perfectly. If they visit /post/apple, Flask automatically blocks it and returns a 404 Not Found error!*

Common Converters:

• <string:name> (Default) Accepts any text without a slash.

• <int:id> Accepts positive integers.

• <float:price> Accepts positive floating point values.

7. Step 4: HTTP Methods (GET vs POST) #

# Allow both GET (viewing the page) and POST (submitting a form)
@app.route('/login', methods=['GET', 'POST'])
def login():
    return "Login Page (Ready to receive data!)"

*(We will learn how to extract the actual form data in Chapter 7).*

8. Backend Workflow: Trailing Slashes #

• @app.route('/about') (No trailing slash): If a user visits /about/ (with a slash), Flask returns a 404 error.

• @app.route('/about/') (With trailing slash): If a user visits /about (no slash), Flask magically redirects them to /about/.

*Best Practice: Always include the trailing slash for major pages (/projects/) so users don't encounter 404 errors if they type it in manually.*

9. Best Practices #

• Route Naming Conventions: The name of your Python function (e.g., def show_post():) should be clear and unique. This function name is internally registered as the "Endpoint Name." Later, we will use this Endpoint Name to dynamically generate links in our HTML, preventing broken links if URL paths change.

10. Common Mistakes #

• Function Name Collisions: A very common beginner mistake is writing two different routes but accidentally giving the Python functions the exact same name.

@app.route('/home')
def index():
    return "Home"

@app.route('/about')
def index(): # ERROR! Function name already exists!
    return "About"

*Flask will crash with an AssertionError. Every View function must have a unique name.*

11. Exercises #

1. 1. Trace the Request flow: A user types http://127.0.0.1:5000/product/99. Explain chronologically how the number 99 is extracted from the URL and passed into the Python logic.

12. Coding Challenges #

• Challenge: In app.py, write a new dynamic route: /calculator/<int:num1>/<int:num2>. The View function should accept both numbers as arguments, add them together, and return a string displaying the result (e.g., "The sum is 10"). Test it in your browser.

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the concept of URL Type Converters in Flask. How do they act as a first layer of security and input validation before the data ever reaches the controller logic?

• Q: Contrast a GET request with a POST request. In a web application, provide a specific scenario where you would explicitly configure a Flask route using methods=['POST'].

15. FAQs #

@app.route('/page/')
@app.route(&#039;/page/<int:num>')
def view_page(num=1):
    return f"Viewing page {num}"

16. Summary #

17. Next Chapter Recommendation #