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diff --git a/app/static/doc/flask-docs/_sources/design.txt b/app/static/doc/flask-docs/_sources/design.txt deleted file mode 100644 index 6ca363a..0000000 --- a/app/static/doc/flask-docs/_sources/design.txt +++ /dev/null @@ -1,191 +0,0 @@ -.. _design: - -Design Decisions in Flask -========================= - -If you are curious why Flask does certain things the way it does and not -differently, this section is for you. This should give you an idea about -some of the design decisions that may appear arbitrary and surprising at -first, especially in direct comparison with other frameworks. - - -The Explicit Application Object -------------------------------- - -A Python web application based on WSGI has to have one central callable -object that implements the actual application. In Flask this is an -instance of the :class:`~flask.Flask` class. Each Flask application has -to create an instance of this class itself and pass it the name of the -module, but why can't Flask do that itself? - -Without such an explicit application object the following code:: - - from flask import Flask - app = Flask(__name__) - - @app.route('/') - def index(): - return 'Hello World!' - -Would look like this instead:: - - from hypothetical_flask import route - - @route('/') - def index(): - return 'Hello World!' - -There are three major reasons for this. The most important one is that -implicit application objects require that there may only be one instance at -the time. There are ways to fake multiple applications with a single -application object, like maintaining a stack of applications, but this -causes some problems I won't outline here in detail. Now the question is: -when does a microframework need more than one application at the same -time? A good example for this is unittesting. When you want to test -something it can be very helpful to create a minimal application to test -specific behavior. When the application object is deleted everything it -allocated will be freed again. - -Another thing that becomes possible when you have an explicit object lying -around in your code is that you can subclass the base class -(:class:`~flask.Flask`) to alter specific behaviour. This would not be -possible without hacks if the object were created ahead of time for you -based on a class that is not exposed to you. - -But there is another very important reason why Flask depends on an -explicit instantiation of that class: the package name. Whenever you -create a Flask instance you usually pass it `__name__` as package name. -Flask depends on that information to properly load resources relative -to your module. With Python's outstanding support for reflection it can -then access the package to figure out where the templates and static files -are stored (see :meth:`~flask.Flask.open_resource`). Now obviously there -are frameworks around that do not need any configuration and will still be -able to load templates relative to your application module. But they have -to use the current working directory for that, which is a very unreliable -way to determine where the application is. The current working directory -is process-wide and if you are running multiple applications in one -process (which could happen in a webserver without you knowing) the paths -will be off. Worse: many webservers do not set the working directory to -the directory of your application but to the document root which does not -have to be the same folder. - -The third reason is "explicit is better than implicit". That object is -your WSGI application, you don't have to remember anything else. If you -want to apply a WSGI middleware, just wrap it and you're done (though -there are better ways to do that so that you do not lose the reference -to the application object :meth:`~flask.Flask.wsgi_app`). - -Furthermore this design makes it possible to use a factory function to -create the application which is very helpful for unittesting and similar -things (:ref:`app-factories`). - -The Routing System ------------------- - -Flask uses the Werkzeug routing system which has was designed to -automatically order routes by complexity. This means that you can declare -routes in arbitrary order and they will still work as expected. This is a -requirement if you want to properly implement decorator based routing -since decorators could be fired in undefined order when the application is -split into multiple modules. - -Another design decision with the Werkzeug routing system is that routes -in Werkzeug try to ensure that there is that URLs are unique. Werkzeug -will go quite far with that in that it will automatically redirect to a -canonical URL if a route is ambiguous. - - -One Template Engine -------------------- - -Flask decides on one template engine: Jinja2. Why doesn't Flask have a -pluggable template engine interface? You can obviously use a different -template engine, but Flask will still configure Jinja2 for you. While -that limitation that Jinja2 is *always* configured will probably go away, -the decision to bundle one template engine and use that will not. - -Template engines are like programming languages and each of those engines -has a certain understanding about how things work. On the surface they -all work the same: you tell the engine to evaluate a template with a set -of variables and take the return value as string. - -But that's about where similarities end. Jinja2 for example has an -extensive filter system, a certain way to do template inheritance, support -for reusable blocks (macros) that can be used from inside templates and -also from Python code, uses Unicode for all operations, supports -iterative template rendering, configurable syntax and more. On the other -hand an engine like Genshi is based on XML stream evaluation, template -inheritance by taking the availability of XPath into account and more. -Mako on the other hand treats templates similar to Python modules. - -When it comes to connecting a template engine with an application or -framework there is more than just rendering templates. For instance, -Flask uses Jinja2's extensive autoescaping support. Also it provides -ways to access macros from Jinja2 templates. - -A template abstraction layer that would not take the unique features of -the template engines away is a science on its own and a too large -undertaking for a microframework like Flask. - -Furthermore extensions can then easily depend on one template language -being present. You can easily use your own templating language, but an -extension could still depend on Jinja itself. - - -Micro with Dependencies ------------------------ - -Why does Flask call itself a microframework and yet it depends on two -libraries (namely Werkzeug and Jinja2). Why shouldn't it? If we look -over to the Ruby side of web development there we have a protocol very -similar to WSGI. Just that it's called Rack there, but besides that it -looks very much like a WSGI rendition for Ruby. But nearly all -applications in Ruby land do not work with Rack directly, but on top of a -library with the same name. This Rack library has two equivalents in -Python: WebOb (formerly Paste) and Werkzeug. Paste is still around but -from my understanding it's sort of deprecated in favour of WebOb. The -development of WebOb and Werkzeug started side by side with similar ideas -in mind: be a good implementation of WSGI for other applications to take -advantage. - -Flask is a framework that takes advantage of the work already done by -Werkzeug to properly interface WSGI (which can be a complex task at -times). Thanks to recent developments in the Python package -infrastructure, packages with dependencies are no longer an issue and -there are very few reasons against having libraries that depend on others. - - -Thread Locals -------------- - -Flask uses thread local objects (context local objects in fact, they -support greenlet contexts as well) for request, session and an extra -object you can put your own things on (:data:`~flask.g`). Why is that and -isn't that a bad idea? - -Yes it is usually not such a bright idea to use thread locals. They cause -troubles for servers that are not based on the concept of threads and make -large applications harder to maintain. However Flask is just not designed -for large applications or asynchronous servers. Flask wants to make it -quick and easy to write a traditional web application. - -Also see the :ref:`becomingbig` section of the documentation for some -inspiration for larger applications based on Flask. - - -What Flask is, What Flask is Not --------------------------------- - -Flask will never have a database layer. It will not have a form library -or anything else in that direction. Flask itself just bridges to Werkzeug -to implement a proper WSGI application and to Jinja2 to handle templating. -It also binds to a few common standard library packages such as logging. -Everything else is up for extensions. - -Why is this the case? Because people have different preferences and -requirements and Flask could not meet those if it would force any of this -into the core. The majority of web applications will need a template -engine in some sort. However not every application needs a SQL database. - -The idea of Flask is to build a good foundation for all applications. -Everything else is up to you or extensions. |