Why does python + pylons "remember" previously specified class variables?

I have a simple form in python + pylons that submits to a controller. However, each page load doesn't seem to be a fresh instantiation of the class. Rather, cla开发者_JAVA技巧ss variables specified on the previous page load are still accessible.

What's going on here? And what's the solution?

A common programmer oversight is that defining a list [] as a default argument or class initialiser is evaluated only once. If you have class variables such as lists, I recommend you initialise them in init. I'll give you an example.

>>> class Example(object):
...     a = []
...     def __init__(self):
...         self.b = []
... 

>>> foo = Example()
>>> bar = Example()

>>> foo.a
[]
>>> bar.a
[]
>>> foo.b
[]
>>> bar.b
[]

>>> foo.a.append(1)
>>> foo.b.append(2)
>>> foo.a
[1]
>>> foo.b
[2]
>>> bar.a
[1]
>>> bar.b
[]

Pylons uses a multi-threaded application server and variables are not cleared from request to request. This is a performance issue, as re-instantiating entire class trees would be expensive. Instead of storing the data returned by the user in a class, use a sessions system (Pylons comes with one or use something like Beaker) or back-end database like SQLAlchemy, SQLObject, or PyMongo.

Additionally, due to the multi-threaded nature of the framework, you should avoid shared objects (like globals) like the plague unless you are very careful to ensure you are using them in a thread-safe way (e.g. read-only). Certain Pylons-supplied objects (request/response) have been written to be thread-local, so don't worry about those.