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diff --git a/app/static/doc/flask-docs/_sources/patterns/lazyloading.txt b/app/static/doc/flask-docs/_sources/patterns/lazyloading.txt new file mode 100644 index 0000000..50ad6fa --- /dev/null +++ b/app/static/doc/flask-docs/_sources/patterns/lazyloading.txt @@ -0,0 +1,104 @@ +Lazily Loading Views +==================== + +Flask is usually used with the decorators. Decorators are simple and you +have the URL right next to the function that is called for that specific +URL. However there is a downside to this approach: it means all your code +that uses decorators has to be imported upfront or Flask will never +actually find your function. + +This can be a problem if your application has to import quick. It might +have to do that on systems like Google's App Engine or other systems. So +if you suddenly notice that your application outgrows this approach you +can fall back to a centralized URL mapping. + +The system that enables having a central URL map is the +:meth:`~flask.Flask.add_url_rule` function. Instead of using decorators, +you have a file that sets up the application with all URLs. + +Converting to Centralized URL Map +--------------------------------- + +Imagine the current application looks somewhat like this:: + + from flask import Flask + app = Flask(__name__) + + @app.route('/') + def index(): + pass + + @app.route('/user/<username>') + def user(username): + pass + +Then the centralized approach you would have one file with the views +(`views.py`) but without any decorator:: + + def index(): + pass + + def user(username): + pass + +And then a file that sets up an application which maps the functions to +URLs:: + + from flask import Flask + from yourapplication import views + app = Flask(__name__) + app.add_url_rule('/', view_func=views.index) + app.add_url_rule('/user/<username>', view_func=views.user) + +Loading Late +------------ + +So far we only split up the views and the routing, but the module is still +loaded upfront. The trick to actually load the view function as needed. +This can be accomplished with a helper class that behaves just like a +function but internally imports the real function on first use:: + + from werkzeug import import_string, cached_property + + class LazyView(object): + + def __init__(self, import_name): + self.__module__, self.__name__ = import_name.rsplit('.', 1) + self.import_name = import_name + + @cached_property + def view(self): + return import_string(self.import_name) + + def __call__(self, *args, **kwargs): + return self.view(*args, **kwargs) + +What's important here is is that `__module__` and `__name__` are properly +set. This is used by Flask internally to figure out how to name the +URL rules in case you don't provide a name for the rule yourself. + +Then you can define your central place to combine the views like this:: + + from flask import Flask + from yourapplication.helpers import LazyView + app = Flask(__name__) + app.add_url_rule('/', + view_func=LazyView('yourapplication.views.index')) + app.add_url_rule('/user/<username>', + view_func=LazyView('yourapplication.views.user')) + +You can further optimize this in terms of amount of keystrokes needed to +write this by having a function that calls into +:meth:`~flask.Flask.add_url_rule` by prefixing a string with the project +name and a dot, and by wrapping `view_func` in a `LazyView` as needed:: + + def url(url_rule, import_name, **options): + view = LazyView('yourapplication.' + import_name) + app.add_url_rule(url_rule, view_func=view, **options) + + url('/', 'views.index') + url('/user/<username>', 'views.user') + +One thing to keep in mind is that before and after request handlers have +to be in a file that is imported upfront to work properly on the first +request. The same goes for any kind of remaining decorator. |