Python 2.x gotchas and landmines

Question

The purpose of my question is to strengthen my knowledge base with Python and get a better picture of it, which includes knowing its faults and surprises. To keep things sp

Answer 1

Using class variables when you want instance variables. Most of the time this doesn't cause problems, but if it's a mutable value it causes surprises.

class Foo(object):
    x = {}

But:

>>> f1 = Foo()
>>> f2 = Foo()
>>> f1.x['a'] = 'b'
>>> f2.x
{'a': 'b'}

You almost always want instance variables, which require you to assign inside __init__:

class Foo(object):
    def __init__(self):
        self.x = {}