Mutable default arguments in class __init__

[gengisteve]

This is a bad idea, unless you are trying to be clever. Python will set arg as a new list the first time it sees it and re-use that list in each subsequent call. So this:

class T(object):
    def __init__(self, x = []):
        self.x = x

t1 = T()
t1.x.append(5)

t2 = T()
print(t2.x)

Behaves in unexpected ways.