Advanced patterns for views and routing

Advanced patterns for views and routing

View decorators

Python decorators are functions that are used to transform other functions. When a decorated function is called, the decorator is called instead. The decorator can then take action, modify the arguments, halt execution or call the original function. We can use decorators to wrap views with code we’d like to run before they are executed.

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@decorator_function
def decorated():
    pass

If you’ve gone through the Flask tutorial, the syntax in this code block might look familiar to you. @app.route is a decorator used to match URLs to view functions in Flask apps.

Let’s take a look at some other decorators you can use in your Flask apps.

Authentication

The Flask-Login extension makes it easy to implement a login system. In addition to handling the details of user authentication, Flask-Login gives us a decorator to restrict certain views to authenticated users: @login_required.

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# app.py

from flask import render_template
from flask_login import login_required, current_user


@app.route('/')
def index():
    return render_template("index.html")

@app.route('/dashboard')
@login_required
def account():
    return render_template("account.html")

Warning

@app.route should always be the outermost view decorator.

Only an authenticated user will be able to access the /dashboard route. We can configure Flask-Login to redirect unauthenticated users to a login page, return an HTTP 401 status or anything else we’d like it to do with them.

Note

Read more about using Flask-Login in the official docs.

Caching

Imagine that an article mentioning our application just appeared on CNN and some other news sites. We’re getting thousands of requests per second. Our homepage makes several trips to the database for each request, so all of this attention is slowing things down to a crawl. How can we speed things up quickly, so all of these visitors don’t miss out on our site?

There are a lot of good answers, but this section is about caching, so we’ll talk about that. Specifically, we’re going to use the Flask-Cache extension. This extension provides us with a decorator that we can use on our index view to cache the response for some period of time.

Flask-Cache can be configured to work with a bunch of different caching backends. A popular choice is Redis, which is easy to set-up and use. Assuming Flask-Cache is already configured, this code block shows what our decorated view would look like.

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# app.py

from flask_cache import Cache
from flask import Flask

app = Flask()

# We'd normally include configuration settings in this call
cache = Cache(app)

@app.route('/')
@cache.cached(timeout=60)
def index():
    [...] # Make a few database calls to get the information we need
    return render_template(
        'index.html',
        latest_posts=latest_posts,
        recent_users=recent_users,
        recent_photos=recent_photos
    )

Now the function will only be run once every 60 seconds, when the cache expires. The response will be saved in our cache and pulled from there for any intervening requests.

Note

Flask-Cache also lets us memoize functions — or cache the result of a function being called with certain arguments. You can even cache computationally expensive Jinja2 template snippets.

Custom decorators

For this section, let’s imagine we have an application that charges users each month. If a user’s account is expired, we’ll redirect them to the billing page and tell them to upgrade.

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# myapp/util.py

from functools import wraps
from datetime import datetime

from flask import flash, redirect, url_for

from flask_login import current_user

def check_expired(func):
    @wraps(func)
    def decorated_function(*args, **kwargs):
        if datetime.utcnow() > current_user.account_expires:
            flash("Your account has expired. Update your billing info.")
            return redirect(url_for('account_billing'))
        return func(*args, **kwargs)

    return decorated_function
10 When a function is decorated with @check_expired, check_expired() is called and the decorated function is passed as a parameter.
11 @wraps is a decorator that does some bookkeeping so that decorated_function() appears as func() for the purposes of documentation and debugging. This makes the behavior of the functions a little more natural.
12 decorated_function will get all of the args and kwargs that were passed to the original view function func(). This is where we check if the user’s account is expired. If it is, we’ll flash a message and redirect them to the billing page.
16 Now that we’ve done what we wanted to do, we run the decorated view function func() with its original arguments.

When we stack decorators, the topmost decorator will run first, then call the next function in line: either the view function or the next decorator. The decorator syntax is just a little syntactic sugar.

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# This code:
@foo
@bar
def one():
    pass

r1 = one()

# is the same as this code:
def two():
    pass

two = foo(bar(two))
r2 = two()

r1 == r2 # True

This code block shows an example using our custom decorator and the @login_required decorator from the Flask-Login extension. We can use multiple decorators by stacking them.

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# myapp/views.py

from flask import render_template

from flask_login import login_required

from . import app
from .util import check_expired

@app.route('/use_app')
@login_required
@check_expired
def use_app():
    """Use our amazing app."""
    # [...]
    return render_template('use_app.html')

@app.route('/account/billing')
@login_required
def account_billing():
    """Update your billing info."""
    # [...]
    return render_template('account/billing.html')

Now when a user tries to access /use_app, check_expired() will make sure that their account hasn’t expired before running the view function.

Note

Read more about what the wraps() function does in the Python docs.

URL Converters

Built-in converters

When you define a route in Flask, you can specify parts of it that will be converted into Python variables and passed to the view function.

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@app.route('/user/<username>')
def profile(username):
    pass

Whatever is in the part of the URL labeled <username> will get passed to the view as the username argument. You can also specify a converter to filter the variable before it’s passed to the view.

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@app.route('/user/id/<int:user_id>')
def profile(user_id):
    pass

In this code block, the URL http://myapp.com/user/id/Q29kZUxlc3NvbiEh will return a 404 status code – not found. This is because the part of the URL that is supposed to be an integer is actually a string.

We could have a second view that looks for a string as well. That would be called for /user/id/Q29kZUxlc3NvbiEh/ while the first would be called for /user/id/124.

This table shows Flask’s built-in URL converters.

string Accepts any text without a slash (the default).
int Accepts integers.
float Like int but for floating point values.
path Like string but accepts slashes.

Custom converters

We can also make custom converters to suit our needs. On Reddit — a popular link sharing site — users create and moderate communities for theme-based discussion and link sharing. Some examples are /r/python and /r/flask, denoted by the path in the URL: reddit.com/r/python and reddit.com/r/flask respectively. An interesting feature of Reddit is that you can view the posts from multiple subreddits as one by seperating the names with a plus-sign in the URL, e.g. reddit.com/r/python+flask.

We can use a custom converter to implement this feature in our own Flask apps. We’ll take an arbitrary number of elements separated by plus-signs, convert them to a list with a ListConverter class and pass the list of elements to the view function.

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# myapp/util.py

from werkzeug.routing import BaseConverter

class ListConverter(BaseConverter):

    def to_python(self, value):
        return value.split('+')

    def to_url(self, values):
        return '+'.join(BaseConverter.to_url(value)
                        for value in values)

We need to define two methods: to_python() and to_url(). As the names suggest, to_python() is used to convert the path in the URL to a Python object that will be passed to the view and to_url() is used by url_for() to convert arguments to their appropriate forms in the URL.

To use our ListConverter, we first have to tell Flask that it exists.

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# /myapp/__init__.py

from flask import Flask

app = Flask(__name__)

from .util import ListConverter

app.url_map.converters['list'] = ListConverter

Warning

This is another chance to run into some circular import problems if your util module has a from . import app line. That’s why I waited until app had been initialized to import ListConverter.

Now we can use our converter just like one of the built-ins. We specified the key in the dictionary as “list” so that’s how we use it in @app.route().

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# myapp/views.py

from . import app

@app.route('/r/<list:subreddits>')
def subreddit_home(subreddits):
    """Show all of the posts for the given subreddits."""
    posts = []
    for subreddit in subreddits:
        posts.extend(subreddit.posts)

    return render_template('/r/index.html', posts=posts)

This should work just like Reddit’s multi-reddit system. This same method can be used to make any URL converter we can dream of.

Summary

  • The @login_required decorator from Flask-Login helps you limit views to authenticated users.
  • The Flask-Cache extension gives you a bunch of decorators to implement various methods of caching.
  • We can develop custom view decorators to help us organize our code and stick to DRY (Don’t Repeat Yourself) coding principles.
  • Custom URL converters can be a great way to implement creative features involving URL’s.