Flask is a simple web framework written in Python.
In this article, we’ll look at how to develop simple Python web apps with Flask.
Unique URLs / Redirection Behavior
Adding a trailing slash to the URL will get us to access to the URL with our without a trailing slash.
For example, if we write:
from flask import Flask
app = Flask(__name__)
@app.route('/foo/')
def projects():
return 'The foo page'
@app.route('/bar')
def about():
return 'The bar page'
We only see ‘The bar page’ when we go to http://localhost:5000/bar.
On the other hand, if we go to http://localhost:5000/foo or http://localhost:5000/foo/, we see ‘The foo page’.
URL Building
We can use the url_for function to show the full URL of each route.
For example, we can write:
from flask import Flask, url_for
from markupsafe import escape
app = Flask(__name__)
@app.route('/')
def index():
return 'index'
@app.route('/login')
def login():
return 'login'
@app.route('/user/<username>')
def profile(username):
return '{}'s profile'.format(escape(username))
with app.test_request_context():
print(url_for('index'))
print(url_for('login'))
print(url_for('login', next='/'))
print(url_for('profile', username='Jane Smith'))
Then in the with block, we print the full paths created from the routes.
We see:
/
/login
/login?next=%2F
/user/Jane%20Smith
url_for('index') returns ‘/‘.
url_for(‘login’) returns '/login' .
url_for(‘login’, next=’/’) returns ‘/login?next=%2F’ .
And url_for(‘profile’, username=’Jane Smith’) returns /user/Jane%20Smith .
We just pass in the function name into url_for as a string, and we get back the URL constructed from it.
HTTP Methods
A route function can take requests made with different HTTP methods.
We can restrict this with the methods parameter we pass into @app.route .
For example, we can write:
from flask import request, Flask
app = Flask(__name__)
@app.route('/login', methods=['GET', 'POST'])
def login():
if request.method == 'POST':
return 'logged in'
else:
return 'login form'
We set the methods parameter to the [‘GET’, ‘POST’] to restrict the function to only accept GET and POST requests.
Then we can check the request method with the request.method property.
So if we make a POST request to http://127.0.0.1:5000/login, we get ‘logged in‘ returned.
And if we make a GET request to the same URL, we get ‘login form‘ returned.
Static Files
We can serve static files with Flask.
To do that, we write:
app.py
from flask import Flask, url_for
app = Flask(__name__)
@app.route('/')
def hello():
return 'hello world'
with app.test_request_context():
print(url_for('static', filename='style.css'))
static/style.css
body {
font-weight: bold;
}
We print the path to the style.css static file in the static folder with the url_for function.
When we go to http://localhost:5000/static/style.css in the browser, we should see:
body {
font-weight: bold;
}
displayed.
Rendering Templates
To render HTML output, we‘ve to render the content in a template.
To do that, we can use tyhe render_template function by writing:
app.py
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/hello/')
@app.route('/hello/<name>')
def hello(name=None):
return render_template('hello.html', name=name)
templates/hello.html
<!doctype html>
<title>Hello from Flask</title>
{% if name %}
<h1>Hello {{ name }}!</h1>
{% else %}
<h1>Hello, World!</h1>
{% endif %}
We added a route that optionally takes the name parameter.
Then we call render_template with the path of the template in the templates folder.
Then we pass the variables we want to render into the subsequent arguments.
In hello.html , we check if the name variable is present, if it is, then we display the first message with the name ‘s value.
If we go to http://localhost:5000/hello/james, we see Hello james! displayed.
Otherwise, if we go to http://localhost:5000/hello, we see Hello, world.
Conclusion
We add static files and render templates with Flask.