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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/>`_.