使用URL处理器

New in version 0.7.

Flask 0.7 introduces the concept of URL processors. The idea is that you might have a bunch of resources with common parts in the URL that you don’t always explicitly want to provide. For instance you might have a bunch of URLs that have the language code in it but you don’t want to have to handle it in every single function yourself.

URL processors are especially helpful when combined with blueprints. We will handle both application specific URL processors here as well as blueprint specifics.

国际化的应用程序 URL

Consider an application like this:

from flask import Flask, g

app = Flask(__name__)

@app.route('/<lang_code>/')
def index(lang_code):
    g.lang_code = lang_code
    ...

@app.route('/<lang_code>/about')
def about(lang_code):
    g.lang_code = lang_code
    ...

This is an awful lot of repetition as you have to handle the language code setting on the g object yourself in every single function. Sure, a decorator could be used to simplify this, but if you want to generate URLs from one function to another you would have to still provide the language code explicitly which can be annoying.

For the latter, this is where url_defaults() functions come in. They can automatically inject values into a call for url_for() automatically. The code below checks if the language code is not yet in the dictionary of URL values and if the endpoint wants a value named 'lang_code':

@app.url_defaults
def add_language_code(endpoint, values):
    if 'lang_code' in values or not g.lang_code:
        return
    if app.url_map.is_endpoint_expecting(endpoint, 'lang_code'):
        values['lang_code'] = g.lang_code

URL映射的is_endpoint_expecting()方法可以用来识别是否可以给末端的函数提供一个多国语言代号码。

该函数相反函数是url_value_preprocessor()。它们在请求成功匹配并且能够执行针对 URL 值的代码时立即执行。The idea is that they pull information out of the values dictionary and put it somewhere else:

@app.url_value_preprocessor
def pull_lang_code(endpoint, values):
    g.lang_code = values.pop('lang_code', None)

That way you no longer have to do the lang_code assignment to g in every function. 你可以通过编写自己的装饰器进一步改进它，这些装饰器给URL添加语言代码前缀，但是使用蓝图会更优雅一些。Once the 'lang_code' is popped from the values dictionary and it will no longer be forwarded to the view function reducing the code to this:

from flask import Flask, g

app = Flask(__name__)

@app.url_defaults
def add_language_code(endpoint, values):
    if 'lang_code' in values or not g.lang_code:
        return
    if app.url_map.is_endpoint_expecting(endpoint, 'lang_code'):
        values['lang_code'] = g.lang_code

@app.url_value_preprocessor
def pull_lang_code(endpoint, values):
    g.lang_code = values.pop('lang_code', None)

@app.route('/<lang_code>/')
def index():
    ...

@app.route('/<lang_code>/about')
def about():
    ...

国际化的Blueprint URL

因为 Blueprint 能够自动地为所有 URL 添加一个相同的字符串作为前缀，所以自动处理这些函数变得非常简单。Furthermore blueprints can have per-blueprint URL processors which removes a whole lot of logic from the url_defaults() function because it no longer has to check if the URL is really interested in a 'lang_code' parameter:

from flask import Blueprint, g

bp = Blueprint('frontend', __name__, url_prefix='/<lang_code>')

@bp.url_defaults
def add_language_code(endpoint, values):
    values.setdefault('lang_code', g.lang_code)

@bp.url_value_preprocessor
def pull_lang_code(endpoint, values):
    g.lang_code = values.pop('lang_code')

@bp.route('/')
def index():
    ...

@bp.route('/about')
def about():
    ...