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Foreword for Experienced Programmers
====================================
This chapter is for programmers who have worked with other frameworks in the
past, and who may have more specific or esoteric concerns that the typical
user.
Threads in Flask
----------------
One of the design decisions with Flask was that simple tasks should be simple;
they should not take a lot of code and yet they should not limit you. Because
of that we made 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.
Web Development is Dangerous
----------------------------
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 to account for the Unicode differences in Python 3.
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 <http://lucumr.pocoo.org/2011/1/22/forwards-compatible-python/>`_.

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What does "micro" mean? What does "micro" mean?
----------------------- -----------------------
Flask considers the "micro" in microframework to refer not only to the “Micro” does not mean that your whole web application has to fit into
simplicity and small size of the framework, but also to the fact that it does a single Python file (although it certainly can). Nor does it mean
not make many decisions for you. While Flask does pick a templating engine that Flask is lacking in functionality. The "micro" in microframework
for you, we won't make such decisions for your datastore or other parts. means Flask aims to keep the core simple but extensible. Flask won't make
many decisions for you, such as what database to use. Those decisions that
it does make, such as what templating engine to use, are easy to change.
Everything else is up to you, so that Flask can be everything you need
and nothing you don't.
However, to us the term “micro” does not mean that the whole implementation By default, Flask does not include a database abstraction layer, form
has to fit into a single Python file. validation or anything else where different libraries already exist that can
handle that. Instead, FLask extensions add such functionality to your
application as if it was implemented in Flask itself. Numerous extensions
provide database integration, form validation, upload handling, various open
authentication technologies, and more. Flask may be "micro", but the
possibilities are endless.
One of the design decisions with Flask was that simple tasks should be Convention over Configuration
simple; they should not take a lot of code and yet they should not limit you. -----------------------------
Because of that we made 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 Flask is based on convention over configuration, which means that many things
many things are preconfigured. For example, by convention templates and are preconfigured. For example, by convention templates and static files are
static files are stored in subdirectories within the application's Python source tree. stored in subdirectories within the application's Python source tree. While
While this can be changed you usually don't have to. this can be changed you usually don't have to. We want to minimize the time
you need to spend in order to get up and running, without assuming things
about your needs.
The main reason Flask is called a "microframework" is the idea Growing Up
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 supports
extensions to add such functionality to 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 Since Flask is based on a very solid foundation there is not a lot of code in
in Flask itself. As such it's easy to adapt even for large applications Flask itself. As such it's easy to adapt even for large applications and we
and we are making sure that you can either configure it as much as are making sure that you can either configure it as much as possible by
possible by subclassing things or by forking the entire codebase. If you subclassing things or by forking the entire codebase. If you are interested
are interested in that, check out the :ref:`becomingbig` chapter. in that, check out the :ref:`becomingbig` chapter.
If you are curious about the Flask design principles, head over to the If you are curious about the Flask design principles, head over to the section
section about :ref:`design`. about :ref:`design`.
Web Development is Dangerous For the Stalwart and Wizened...
---------------------------- -------------------------------
If you write a web application, you are probably allowing users to register If you're more curious about the minutiae of Flask's implementation, and
and leave their data on your server. The users are entrusting you with data. whether its structure is right for your needs, read the
And even if you are the only user that might leave data in your application, :ref:`advanced_foreword`.
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 to account for the Unicode differences in Python 3.
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 <http://lucumr.pocoo.org/2011/1/22/forwards-compatible-python/>`_.