视图装饰器

Python 有一个非常有趣的特性，称为函数装饰器。这使得在 Web 应用中实现一些非常巧妙的操作成为可能。由于 Flask 中的每个视图都是一个函数，装饰器可以用于为一个或多个函数注入额外的功能。你可能已经使用过 route() 装饰器。不过，也存在需要实现自定义装饰器的情形。例如，设想你有一个视图，它只应由已登录的用户访问。如果某个用户访问站点时尚未登录，则应将其重定向到登录页面。这正是装饰器能够提供出色解决方案的一个典型用例。

Login Required Decorator

So let's implement such a decorator. A decorator is a function that wraps and replaces another function. Since the original function is replaced, you need to remember to copy the original function's information to the new function. Use functools.wraps() to handle this for you.

This example assumes that the login page is called 'login' and that the current user is stored in g.user and is None if there is no-one logged in.

from functools import wraps
from flask import g, request, redirect, url_for

def login_required(f):
    @wraps(f)
    def decorated_function(*args, **kwargs):
        if g.user is None:
            return redirect(url_for('login', next=request.url))
        return f(*args, **kwargs)
    return decorated_function

To use the decorator, apply it as innermost decorator to a view function. When applying further decorators, always remember that the route() decorator is the outermost.

@app.route('/secret_page')
@login_required
def secret_page():
    pass

备注

The next value will exist in request.args after a GET request for the login page. You'll have to pass it along when sending the POST request from the login form. You can do this with a hidden input tag, then retrieve it from request.form when logging the user in.

<input type="hidden" value="{{ request.args.get('next', '') }}"/>

Caching Decorator

Imagine you have a view function that does an expensive calculation and because of that you would like to cache the generated results for a certain amount of time. A decorator would be nice for that. We're assuming you have set up a cache like mentioned in 缓存.

Here is an example cache function. It generates the cache key from a specific prefix (actually a format string) and the current path of the request. Notice that we are using a function that first creates the decorator that then decorates the function. Sounds awful? Unfortunately it is a little bit more complex, but the code should still be straightforward to read.

The decorated function will then work as follows

1. get the unique cache key for the current request based on the current path.

2. get the value for that key from the cache. If the cache returned something we will return that value.

3. otherwise the original function is called and the return value is stored in the cache for the timeout provided (by default 5 minutes).

Here the code:

from functools import wraps
from flask import request

def cached(timeout=5 * 60, key='view/{}'):
    def decorator(f):
        @wraps(f)
        def decorated_function(*args, **kwargs):
            cache_key = key.format(request.path)
            rv = cache.get(cache_key)
            if rv is not None:
                return rv
            rv = f(*args, **kwargs)
            cache.set(cache_key, rv, timeout=timeout)
            return rv
        return decorated_function
    return decorator

Notice that this assumes an instantiated cache object is available, see 缓存.

Templating Decorator

A common pattern invented by the TurboGears guys a while back is a templating decorator. The idea of that decorator is that you return a dictionary with the values passed to the template from the view function and the template is automatically rendered. With that, the following three examples do exactly the same:

@app.route('/')
def index():
    return render_template('index.html', value=42)

@app.route('/')
@templated('index.html')
def index():
    return dict(value=42)

@app.route('/')
@templated()
def index():
    return dict(value=42)

As you can see, if no template name is provided it will use the endpoint of the URL map with dots converted to slashes + '.html'. Otherwise the provided template name is used. When the decorated function returns, the dictionary returned is passed to the template rendering function. If None is returned, an empty dictionary is assumed, if something else than a dictionary is returned we return it from the function unchanged. That way you can still use the redirect function or return simple strings.

Here is the code for that decorator:

from functools import wraps
from flask import request, render_template

def templated(template=None):
    def decorator(f):
        @wraps(f)
        def decorated_function(*args, **kwargs):
            template_name = template
            if template_name is None:
                template_name = f"{request.endpoint.replace('.', '/')}.html"
            ctx = f(*args, **kwargs)
            if ctx is None:
                ctx = {}
            elif not isinstance(ctx, dict):
                return ctx
            return render_template(template_name, **ctx)
        return decorated_function
    return decorator

Endpoint Decorator

When you want to use the werkzeug routing system for more flexibility you need to map the endpoint as defined in the Rule to a view function. This is possible with this decorator. For example:

from flask import Flask
from werkzeug.routing import Rule

app = Flask(__name__)
app.url_map.add(Rule('/', endpoint='index'))

@app.endpoint('index')
def my_index():
    return "Hello world"