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Foreword

Read this before you get started with Flask. This hopefully answers some questions about the purpose and goals of the project, and when you should or should not be using it.

What does "micro" mean?

To me, the "micro" in microframework refers not only to the simplicity and small size of the framework, but also the fact that it does not make much decisions for you. While Flask does pick a templating engine for you, we won't make such decisions for your datastore or other parts.

For us however the term “micro” does not mean that the whole implementation has to fit into a single Python file.

One of the design decisions with Flask was that simple tasks should be simple and not take up a lot of code and yet not limit yourself. Because of that we took a few design choices that some people might find surprising or unorthodox. For example, Flask uses thread-local objects internally so that you don't have to pass objects around from function to function within a request in order to stay threadsafe. While this is a really easy approach and saves you a lot of time, it might also cause some troubles for very large applications because changes on these thread-local objects can happen anywhere in the same thread. In order to solve these problems we don't hide the thread locals for you but instead embrace them and provide you with a lot of tools to make it as pleasant as possible to work with them.

Flask is also based on convention over configuration, which means that many things are preconfigured. For example, by convention, templates and static files are in subdirectories within the Python source tree of the application. While this can be changed you usually don't have to.

The main reason however why Flask is called a "microframework" is the idea to keep the core simple but extensible. There is no database abstraction layer, no form validation or anything else where different libraries already exist that can handle that. However Flask knows the concept of extensions that can add this functionality into your application as if it was implemented in Flask itself. There are currently extensions for object relational mappers, form validation, upload handling, various open authentication technologies and more.

Since Flask is based on a very solid foundation there is not a lot of code in Flask itself. As such it's easy to adapt even for lage applications and we are making sure that you can either configure it as much as possible by subclassing things or by forking the entire codebase. If you are interested in that, check out the :ref:`becomingbig` chapter.

If you are curious about the Flask design principles, head over to the section about :ref:`design`.

Web Development is Dangerous

I'm not joking. Well, maybe a little. If you write a web application, you are probably allowing users to register and leave their data on your server. The users are entrusting you with data. And even if you are the only user that might leave data in your application, you still want that data to be stored securely.

Unfortunately, there are many ways the security of a web application can be compromised. Flask protects you against one of the most common security problems of modern web applications: cross-site scripting (XSS). Unless you deliberately mark insecure HTML as secure, Flask and the underlying Jinja2 template engine have you covered. But there are many more ways to cause security problems.

The documentation will warn you about aspects of web development that require attention to security. Some of these security concerns are far more complex than one might think, and we all sometimes underestimate the likelihood that a vulnerability will be exploited, until a clever attacker figures out a way to exploit our applications. And don't think that your application is not important enough to attract an attacker. Depending on the kind of attack, chances are that automated bots are probing for ways to fill your database with spam, links to malicious software, and the like.

So always keep security in mind when doing web development.

The Status of Python 3

Currently the Python community is in the process of improving libraries to support the new iteration of the Python programming language. While the situation is greatly improving there are still some issues that make it hard for us to switch over to Python 3 just now. These problems are partially caused by changes in the language that went unreviewed for too long, partially also because we have not quite worked out how the lower level API should change for the unicode differences in Python3.

Werkzeug and Flask will be ported to Python 3 as soon as a solution for the changes is found, and we will provide helpful tips how to upgrade existing applications to Python 3. Until then, we strongly recommend using Python 2.6 and 2.7 with activated Python 3 warnings during development. If you plan on upgrading to Python 3 in the near future we strongly recommend that you read How to write forwards compatible Python code.

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