Question

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, class variables specified on the previous page load are still accessible.

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

Answer 1

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
[]