diff --git a/docs/patterns/index.rst b/docs/patterns/index.rst index ed231163..4233808b 100644 --- a/docs/patterns/index.rst +++ b/docs/patterns/index.rst @@ -19,6 +19,7 @@ Snippet Archives `_. packages appfactories appdispatch + urlprocessors distribute fabric sqlite3 diff --git a/docs/patterns/urlprocessors.rst b/docs/patterns/urlprocessors.rst new file mode 100644 index 00000000..f00211a6 --- /dev/null +++ b/docs/patterns/urlprocessors.rst @@ -0,0 +1,126 @@ +Using URL Processors +==================== + +.. versionadded:: 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('//') + def index(lang_code): + g.lang_code = lang_code + ... + + @app.route('//about') + def about(lang_code): + g.lang_code = lang_code + ... + +This is an awful lot of reptition as you have to handle the language code +setting on the :data:`~flask.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 :func:`~flask.Flask.url_defaults` functions +come in. They can automatically inject values into a call for +:func:`~flask.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 :meth:`~werkzeug.routing.Map.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 +:meth:`~flask.Flask.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 +:data:`~flask.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('//') + def index(): + ... + + @app.route('//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 :meth:`~flask.Flask.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='/') + + @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(): + ...