Using URL Processors¶

+

+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.

+

Internationalized Application URLs¶

+

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
+

+

+

The method is_endpoint_expecting() of the URL +map can be used to figure out if it would make sense to provide a language +code for the given endpoint.

+

The reverse of that function are +url_value_preprocessor()s. They are executed right +after the request was matched and can execute code based on the URL +values. 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 assigment to +g in every function. You can further improve that by +writing your own decorator that prefixes URLs with the language code, but +the more beautiful solution is using a blueprint. 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():
+    ...
+

+

+

Internationalized Blueprint URLs¶

+

Because blueprints can automatically prefix all URLs with a common string +it’s easy to automatically do that for every function. 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():
+    ...
+

+

+

+ + +

Using URL Processors¶

Internationalized Application URLs¶

Internationalized Blueprint URLs¶

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Using URL Processors¶

Internationalized Application URLs¶

Internationalized Blueprint URLs¶

Table Of Contents

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