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Routing and view functions - advanced techniques
Welcome again, @name! Darwin here, ready to show you advanced routing techniques in Flask! 🛤️🔀
In the previous lesson you created a simple Flask application with basic routes. Now you'll learn professional techniques used in production applications! 🚀
Safari Analogy: Routing is like a network of safari trails - you need to be able to redirect tourists (users), handle different routes, react to errors (getting lost), and build maps (URL building)! 🗺️🧭
Reminder - routing basics
1from flask import Flask
2
3app = Flask(__name__)
4
5@app.route('/')
6def home():
7 return "Home page"
8
9@app.route('/species/<int:species_id>')
10def show_species(species_id):
11 return f"Species ID: {species_id}"Route = decorator `@app.route()` mapping URL → view function.
Now you'll learn advanced techniques! 🎯
Multiple routes for one function
You can assign multiple URLs to the same function:
1@app.route('/')
2@app.route('/home')
3@app.route('/index')
4def home():
5 return "<h1>Safari Database Home Page</h1>"The user can visit:
- `/` → `home()` executes
- `/home` → `home()` executes
- `/index` → `home()` executes
Why? Multiple possible entries to the same content - more convenient for users! 🚪🚪🚪
Analogy: It's like multiple entrances to the same habitat - tourists can enter from the north, south or east, but they all see the same lions! 🦁
Trailing slash - difference between / and without /
Flask distinguishes URLs with `/` at the end and without:
1# With trailing slash (default)
2@app.route('/species/')
3def list_species():
4 return "Species list"
5
6# Without trailing slash
7@app.route('/about')
8def about():
9 return "About the system"How does it work?
With trailing slash (`/species/`):
- `/species/` → ✅ works
- `/species` → ✅ 301 redirect to `/species/`
Without trailing slash (`/about`):
- `/about` → ✅ works
- `/about/` → ❌ 404 Not Found
Best practice: Use with trailing slash for "folders" (collections), without slash for "files" (individual resources):
1@app.route('/species/') # List (folder)
2@app.route('/species/<int:id>') # Single item (file)HTTP Methods - details
Flask allows handling different HTTP methods in the same route:
1from flask import request
2
3@app.route('/species', methods=['GET', 'POST', 'PUT', 'DELETE'])
4def species_handler():
5 if request.method == 'GET':
6 return "Get species list"
7
8 elif request.method == 'POST':
9 return "Create new species"
10
11 elif request.method == 'PUT':
12 return "Update species"
13
14 elif request.method == 'DELETE':
15 return "Delete species"Better approach - separate functions:
1@app.route('/species', methods=['GET'])
2def list_species():
3 return "Species list"
4
5@app.route('/species', methods=['POST'])
6def create_species():
7 return "Species created"Or even better - use shortcut decorators:
1@app.get('/species')
2def list_species():
3 return "Species list (GET)"
4
5@app.post('/species')
6def create_species():
7 return "Species created (POST)"
8
9@app.put('/species/<int:id>')
10def update_species(id):
11 return f"Species {id} updated (PUT)"
12
13@app.delete('/species/<int:id>')
14def delete_species(id):
15 return f"Species {id} deleted (DELETE)"Analogy: Different HTTP methods are like different activities on safari - GET is observation (safe), POST is adding a new species to the database (state change), DELETE is removing an entry! 🔍➕❌
Request object - accessing data
The `request` object contains all information about the HTTP request:
1from flask import request
2
3@app.route('/search')
4def search():
5 # Query parameters (?name=lion&habitat=savanna)
6 species_name = request.args.get('name')
7 habitat = request.args.get('habitat')
8 page = request.args.get('page', 1, type=int) # Default 1
9
10 return f"""
11 <h1>Search</h1>
12 <p>Species: {species_name}</p>
13 <p>Habitat: {habitat}</p>
14 <p>Page: {page}</p>
15 """
16
17@app.route('/add-species', methods=['POST'])
18def add_species():
19 # Form data (from HTML form)
20 name = request.form.get('name')
21 population = request.form.get('population', type=int)
22
23 return f"Added: {name} (population: {population})"
24
25@app.route('/api/species', methods=['POST'])
26def api_create_species():
27 # JSON data (from fetch/API)
28 data = request.get_json()
29 name = data.get('name')
30 population = data.get('population')
31
32 return {"message": f"Created {name}", "id": 123}
33
34@app.route('/info')
35def request_info():
36 return f"""
37 <h1>Request Information</h1>
38 <p><strong>URL:</strong> {request.url}</p>
39 <p><strong>Method:</strong> {request.method}</p>
40 <p><strong>Path:</strong> {request.path}</p>
41 <p><strong>User-Agent:</strong> {request.user_agent}</p>
42 <p><strong>IP:</strong> {request.remote_addr}</p>
43 <p><strong>Cookies:</strong> {request.cookies}</p>
44 <p><strong>Headers:</strong> {dict(request.headers)}</p>
45 """Useful `request` attributes:
- `request.args` - query parameters (?key=value)
- `request.form` - form data (POST)
- `request.get_json()` - JSON data
- `request.files` - uploaded files
- `request.cookies` - cookies
- `request.headers` - HTTP headers
- `request.method` - HTTP method (GET, POST, etc.)
- `request.url` - full URL
- `request.path` - URL path
- `request.remote_addr` - client IP
Example - Safari search engine:
1@app.route('/search')
2def search():
3 query = request.args.get('q', '')
4 filter_type = request.args.get('type', 'all')
5 sort = request.args.get('sort', 'name')
6
7 # Search simulation
8 results = [
9 {"name": "Lion", "type": "mammal"},
10 {"name": "Cheetah", "type": "mammal"},
11 ]
12
13 # Filter by type
14 if filter_type != 'all':
15 results = [r for r in results if r['type'] == filter_type]
16
17 html = f"<h1>Results for: {query}</h1>"
18 html += f"<p>Type: {filter_type}, Sort: {sort}</p>"
19
20 for result in results:
21 html += f"<p>🦁 {result['name']} ({result['type']})</p>"
22
23 return htmlTest:
- `/search?q=lion`
- `/search?q=lion&type=mammal&sort=population`
Response object - returning responses
Flask automatically converts returned values to Response objects:
1from flask import make_response, jsonify
2
3# Simple string (status 200)
4@app.route('/hello')
5def hello():
6 return "Hello Safari!"
7
8# String + status code
9@app.route('/created')
10def created():
11 return "Species created!", 201
12
13# String + status + headers
14@app.route('/custom')
15def custom():
16 return "Custom response", 200, {'X-Custom-Header': 'Safari'}
17
18# JSON response
19@app.route('/api/species/<int:id>')
20def api_species(id):
21 species = {"id": id, "name": "Lion", "population": 120}
22 return jsonify(species)
23
24# make_response() - more control
25@app.route('/cookie')
26def set_cookie():
27 response = make_response("Cookie set!")
28 response.set_cookie('user_id', '123', max_age=3600)
29 return responseReturn format:
- `return "text"` → 200 OK
- `return "text", 201` → 201 Created
- `return "text", 404` → 404 Not Found
- `return jsonify(data)` → JSON response
- `return make_response(...)` → custom Response
Redirects
Use `redirect()` to redirect the user to another URL:
1from flask import redirect, url_for
2
3@app.route('/old-species')
4def old_species():
5 # Redirect to new URL
6 return redirect('/species')
7
8@app.route('/species/<int:id>')
9def show_species(id):
10 # If species doesn't exist, redirect to list
11 species = get_species_by_id(id)
12
13 if species is None:
14 return redirect('/species')
15
16 return f"<h1>{species['name']}</h1>"
17
18@app.route('/login', methods=['POST'])
19def login():
20 username = request.form.get('username')
21 password = request.form.get('password')
22
23 if authenticate(username, password):
24 # Success - redirect to dashboard
25 return redirect('/dashboard')
26 else:
27 # Error - redirect back to login
28 return redirect('/login?error=invalid')Analogy: A redirect is like a safari route change - if a trail is closed (species doesn't exist), the guide redirects the group to another route (species list)! 🔀🛤️
url_for() - building URLs
`url_for()` builds URLs based on function name instead of hardcoded URLs:
1from flask import url_for
2
3@app.route('/')
4def home():
5 # Instead of hardcoded '/species'
6 species_url = url_for('list_species')
7
8 # Instead of hardcoded '/species/1'
9 lion_url = url_for('show_species', species_id=1)
10
11 return f"""
12 <h1>Safari Database</h1>
13 <a href="{species_url}">Species list</a> |
14 <a href="{lion_url}">See lion</a>
15 """
16
17@app.route('/species')
18def list_species():
19 return "<h1>Species list</h1>"
20
21@app.route('/species/<int:species_id>')
22def show_species(species_id):
23 return f"<h1>Species ID: {species_id}</h1>"Why `url_for()`? ✅ Centralized routing - change URL in one place ✅ Safer - Flask automatically escapes parameters ✅ More readable - you use function names instead of URLs
Example with query parameters:
1@app.route('/search')
2def search():
3 return "Search results"
4
5# Build URL with query params
6url = url_for('search', q='lion', type='mammal')
7# Result: /search?q=lion&type=mammalRedirect with `url_for()`:
1@app.route('/old-url')
2def old_url():
3 return redirect(url_for('list_species'))Error Handlers
Create custom error pages for 404, 500, etc.:
1# 404 - Not Found
2@app.errorhandler(404)
3def page_not_found(error):
4 return """
5 <!DOCTYPE html>
6 <html>
7 <head>
8 <title>404 - Safari Database</title>
9 <style>
10 body {
11 font-family: Arial, sans-serif;
12 text-align: center;
13 padding: 100px;
14 background-color: #f4f1de;
15 }
16 h1 { color: #d62828; font-size: 72px; }
17 </style>
18 </head>
19 <body>
20 <h1>404</h1>
21 <h2>🦁 Species not found!</h2>
22 <p>The requested resource does not exist in Safari Database.</p>
23 <a href="/">← Back to home page</a>
24 </body>
25 </html>
26 """, 404
27
28# 500 - Internal Server Error
29@app.errorhandler(500)
30def internal_error(error):
31 return """
32 <h1>500 - Server Error</h1>
33 <p>Something went wrong. Our lions are working on the fix!</p>
34 <a href="/">← Back to home page</a>
35 """, 500
36
37# Custom exception
38class SpeciesNotFoundError(Exception):
39 pass
40
41@app.errorhandler(SpeciesNotFoundError)
42def handle_species_not_found(error):
43 return f"<h1>Species not found</h1><p>{error}</p>", 404
44
45@app.route('/species/<int:id>')
46def show_species(id):
47 species = get_species_by_id(id)
48
49 if species is None:
50 raise SpeciesNotFoundError(f"Species with ID {id} does not exist")
51
52 return f"<h1>{species['name']}</h1>"Analogy: Error handlers are like emergency procedures on safari - when a tourist gets lost (404) or there's a vehicle breakdown (500), the guide knows how to react! 🚨🦁
Before/After Request Hooks
Flask allows executing code before and after each request:
1import time
2
3# Before each request
4@app.before_request
5def before_request():
6 # Save start time
7 request.start_time = time.time()
8
9 # Log request
10 print(f"🔵 {request.method} {request.path}")
11
12# After each request
13@app.after_request
14def after_request(response):
15 # Calculate execution time
16 if hasattr(request, 'start_time'):
17 duration = time.time() - request.start_time
18 print(f"✅ Execution time: {duration:.3f}s")
19
20 # Add custom header
21 response.headers['X-Safari-Database'] = 'v1.0'
22
23 return response
24
25# After each request (even when error occurs)
26@app.teardown_request
27def teardown_request(exception=None):
28 if exception:
29 print(f"❌ An error occurred: {exception}")
30
31 # Close database connection
32 # db.close()Use cases:
- 🔐 Authentication (check if user is logged in)
- 📊 Request logging
- ⏱️ Measuring execution time
- 🗄️ Database connection/closing
- 🔒 Security headers
Analogy: Hooks are like pre/post-safari procedures - before: check equipment, count tourists; after: record observations, make sure everyone returned! 📋✅
Practical example - Safari Database v2
Let's create an improved version of Safari Database using all the techniques we've learned! 🦁
1# app.py
2from flask import Flask, request, redirect, url_for, make_response, jsonify
3import time
4
5app = Flask(__name__)
6
7# Test data
8species_db = {
9 1: {"id": 1, "name": "Lion", "scientific_name": "Panthera leo", "population": 120},
10 2: {"id": 2, "name": "Elephant", "scientific_name": "Loxodonta africana", "population": 450},
11 3: {"id": 3, "name": "Cheetah", "scientific_name": "Acinonyx jubatus", "population": 7100},
12}
13next_id = 4
14
15# --- Hooks ---
16@app.before_request
17def log_request():
18 request.start_time = time.time()
19 print(f"🔵 [{request.method}] {request.path}")
20
21@app.after_request
22def log_response(response):
23 if hasattr(request, 'start_time'):
24 duration = time.time() - request.start_time
25 print(f"✅ Status: {response.status_code} | Time: {duration:.3f}s")
26 return response
27
28# --- Routes ---
29@app.route('/')
30@app.route('/home')
31def home():
32 return f"""
33 <!DOCTYPE html>
34 <html>
35 <head>
36 <title>Safari Database</title>
37 <style>
38 body {{ font-family: Arial; max-width: 800px; margin: 50px auto; background: #f4f1de; }}
39 h1 {{ color: #2c5f2d; }}
40 nav {{ margin: 20px 0; }}
41 nav a {{ margin-right: 15px; color: #2c5f2d; text-decoration: none; font-weight: bold; }}
42 nav a:hover {{ text-decoration: underline; }}
43 </style>
44 </head>
45 <body>
46 <h1>🦁 Safari Database</h1>
47 <nav>
48 <a href="{url_for('home')}">Home</a>
49 <a href="{url_for('list_species')}">Species list</a>
50 <a href="{url_for('add_species_form')}">Add species</a>
51 <a href="{url_for('search')}">Search</a>
52 </nav>
53 <h2>Welcome to Safari Database!</h2>
54 <p>The database contains <strong>{len(species_db)} species</strong>.</p>
55 <p><a href="{url_for('list_species')}">See all →</a></p>
56 </body>
57 </html>
58 """
59
60@app.get('/species')
61def list_species():
62 species_html = ""
63 for spec in species_db.values():
64 species_html += f"""
65 <div style="background: white; padding: 15px; margin: 10px 0; border-radius: 8px;">
66 <h3>{spec['name']}</h3>
67 <p><strong>Population:</strong> {spec['population']}</p>
68 <a href="{url_for('show_species', species_id=spec['id'])}">Details</a> |
69 <a href="{url_for('delete_species', species_id=spec['id'])}" style="color: red;">Delete</a>
70 </div>
71 """
72
73 return f"""
74 <!DOCTYPE html>
75 <html>
76 <head><title>Species list</title></head>
77 <body style="font-family: Arial; max-width: 800px; margin: 50px auto; background: #f4f1de;">
78 <h1>🦁 Species list ({len(species_db)})</h1>
79 {species_html}
80 <p><a href="{url_for('home')}">← Back</a></p>
81 </body>
82 </html>
83 """
84
85@app.route('/species/<int:species_id>')
86def show_species(species_id):
87 spec = species_db.get(species_id)
88
89 if spec is None:
90 # Use custom error handler
91 return f"<h1>404</h1><p>Species {species_id} does not exist</p>", 404
92
93 return f"""
94 <!DOCTYPE html>
95 <html>
96 <head><title>{spec['name']}</title></head>
97 <body style="font-family: Arial; max-width: 800px; margin: 50px auto; background: #f4f1de;">
98 <h1>🦁 {spec['name']}</h1>
99 <div style="background: white; padding: 30px; border-radius: 12px;">
100 <p><strong>Scientific name:</strong> {spec['scientific_name']}</p>
101 <p><strong>Population:</strong> {spec['population']} individuals</p>
102 <p><strong>ID:</strong> {spec['id']}</p>
103 </div>
104 <p style="margin-top: 20px;">
105 <a href="{url_for('list_species')}">← List</a> |
106 <a href="{url_for('home')}">← Home</a>
107 </p>
108 </body>
109 </html>
110 """
111
112@app.route('/species/add', methods=['GET'])
113def add_species_form():
114 return f"""
115 <!DOCTYPE html>
116 <html>
117 <head><title>Add species</title></head>
118 <body style="font-family: Arial; max-width: 800px; margin: 50px auto; background: #f4f1de;">
119 <h1>➕ Add new species</h1>
120 <form method="POST" action="{url_for('add_species')}" style="background: white; padding: 30px; border-radius: 12px;">
121 <p><label>Name: <input type="text" name="name" required></label></p>
122 <p><label>Scientific name: <input type="text" name="scientific_name" required></label></p>
123 <p><label>Population: <input type="number" name="population" required></label></p>
124 <button type="submit" style="padding: 10px 20px; background: #2c5f2d; color: white; border: none; border-radius: 4px;">Add</button>
125 </form>
126 <p><a href="{url_for('home')}">← Back</a></p>
127 </body>
128 </html>
129 """
130
131@app.post('/species/add')
132def add_species():
133 global next_id
134
135 name = request.form.get('name')
136 scientific_name = request.form.get('scientific_name')
137 population = request.form.get('population', type=int)
138
139 # Add to database
140 new_species = {
141 "id": next_id,
142 "name": name,
143 "scientific_name": scientific_name,
144 "population": population
145 }
146 species_db[next_id] = new_species
147 next_id += 1
148
149 # Redirect to new species details
150 return redirect(url_for('show_species', species_id=new_species['id']))
151
152@app.route('/species/<int:species_id>/delete')
153def delete_species(species_id):
154 if species_id in species_db:
155 del species_db[species_id]
156
157 # Redirect to list
158 return redirect(url_for('list_species'))
159
160@app.route('/search')
161def search():
162 query = request.args.get('q', '').lower()
163
164 if not query:
165 return f"""
166 <h1>🔍 Search</h1>
167 <form method="GET">
168 <input type="text" name="q" placeholder="Enter species name">
169 <button type="submit">Search</button>
170 </form>
171 <a href="{url_for('home')}">← Back</a>
172 """
173
174 # Search
175 results = [s for s in species_db.values() if query in s['name'].lower()]
176
177 results_html = f"<h2>Found: {len(results)}</h2>"
178 for spec in results:
179 results_html += f"<p>🦁 <a href='{url_for('show_species', species_id=spec['id'])}'>{spec['name']}</a></p>"
180
181 return f"""
182 <h1>🔍 Results for: "{query}"</h1>
183 {results_html}
184 <p><a href="{url_for('search')}">← New search</a></p>
185 """
186
187# --- API Endpoints ---
188@app.get('/api/species')
189def api_list_species():
190 return jsonify(list(species_db.values()))
191
192@app.get('/api/species/<int:species_id>')
193def api_get_species(species_id):
194 spec = species_db.get(species_id)
195 if spec is None:
196 return jsonify({"error": "Not found"}), 404
197 return jsonify(spec)
198
199# --- Error Handlers ---
200@app.errorhandler(404)
201def page_not_found(error):
202 return f"""
203 <div style="text-align: center; padding: 100px; background: #f4f1de;">
204 <h1 style="font-size: 72px; color: #d62828;">404</h1>
205 <h2>🦁 Not found!</h2>
206 <p><a href="{url_for('home')}">← Back to home page</a></p>
207 </div>
208 """, 404
209
210if __name__ == '__main__':
211 print("🦁 Safari Database v2 starting...")
212 app.run(debug=True)Test the application:
- http://localhost:5000/ - home page
- http://localhost:5000/species - species list
- http://localhost:5000/species/1 - lion details
- http://localhost:5000/species/add - add species
- http://localhost:5000/search?q=lion - search
- http://localhost:5000/api/species - API JSON
Summary
In this lesson you learned:
- ✅ Multiple routes for one function
- ✅ Trailing slash (/ vs without /)
- ✅ HTTP methods (GET, POST, PUT, DELETE)
- ✅ Request object (args, form, json, headers)
- ✅ Response object (make_response, jsonify)
- ✅ Redirects (redirect, url_for)
- ✅ URL building with `url_for()`
- ✅ Error handlers (404, 500, custom)
- ✅ Before/After request hooks
- ✅ Practical Safari Database v2 application
Final Safari Analogy: Advanced routing is like a professional safari trail system - you can redirect tourists (`redirect`), handle getting lost (`404`), measure route time (hooks), and build maps from names instead of coordinates (`url_for`)! 🗺️🧭
Next lesson: Darwin will show you Jinja2 templates - we're done with hardcoded HTML strings and starting dynamic templates! 🎨📄