When a Flask app crashes due to missing routes, it means the application is trying to access an endpoint that hasn’t been defined or registered properly. This issue can result in errors like “404 Not Found” or “AttributeError: ‘NoneType’ object has no attribute” if a function is expected to return a response but isn’t properly linked.
🔍 Common Causes & Fixes
1️⃣ Route Not Defined
Cause:
You may be trying to access a route that hasn’t been defined in your Flask app.
Example (Incorrect Code):
from flask import Flask
app = Flask(__name__)
@app.route("/")
def home():
return "Welcome to my app!"
Issue: If you try to access /about, Flask will return a 404 Not Found error.
Fix: Ensure the route is defined:
@app.route("/about")
def about():
return "About Page"
2️⃣ Typos in Route Names
Cause:
A small spelling mistake in the route definition can prevent Flask from recognizing it.
Example (Incorrect Code):
@app.route("/contact-us") # Defined with a hyphen
def contact():
return "Contact Page"
If you try to access /contact_us, it will return 404 Not Found.
Fix: Ensure consistency in naming:
@app.route("/contact_us") # Use an underscore instead
def contact():
return "Contact Page"
3️⃣ Incorrect app.route Decorator Usage
Cause:
Flask routes must be defined with @app.route(). If you forget the decorator, the function won’t be registered.
Example (Incorrect Code):
def services(): # No @app.route() decorator
return "Services Page"
Fix:
@app.route("/services")
def services():
return "Services Page"
4️⃣ Running the Wrong Flask File
Cause:
If you have multiple Python files and are running the wrong one, some routes may not load properly.
Fix:
Make sure you’re running the correct file. If your app is in app.py, start it with:
flask run
Or explicitly set the file:
export FLASK_APP=app.py
flask run
5️⃣ Blueprint Routes Not Registered
Cause:
If you’re using Flask Blueprints, you need to register them properly.
Example (Incorrect Code):
from flask import Flask, Blueprint
app = Flask(__name__)
bp = Blueprint('bp', __name__)
@bp.route("/dashboard")
def dashboard():
return "Dashboard Page"
# Forgot to register Blueprint
Fix: Register the Blueprint:
app.register_blueprint(bp, url_prefix="/admin")
Now, /admin/dashboard will work correctly.
6️⃣ Flask Debug Mode Not Enabled (Missing Errors)
Cause:
If debug mode is off, Flask won’t show useful error messages.
Fix: Enable debug mode:
app.run(debug=True)
7️⃣ __name__ == "__main__" Block Missing
Cause:
Without the if __name__ == "__main__" block, Flask might not start properly.
Fix: Always include:
if __name__ == "__main__":
app.run(debug=True)
🛠 Summary of Fixes
| Issue | Fix |
|---|---|
| Route not defined | Ensure the route exists (@app.route("/path")) |
| Typo in route name | Use consistent naming |
Missing @app.route decorator | Add @app.route("/path") above function |
| Running the wrong file | Set export FLASK_APP=app.py |
| Blueprint not registered | Use app.register_blueprint(bp) |
| Debug mode off | Set app.run(debug=True) |
Missing __name__ == "__main__" block | Ensure proper script execution |