Default function arguments (mutable type)

gistfile1

In [1]: class Foo(object):
   ...:     def __init__(self, bar=[]):
   ...:         self.bar = bar
   ...:
   ...:

In [2]: f1 = Foo()

In [3]: f2 = Foo()

In [4]: f1.bar
Out[4]: []

In [5]: f2.bar
Out[5]: []

In [6]: f1.bar.append(1)

In [7]: f1.bar
Out[7]: [1]

In [8]: f2.bar
Out[8]: [1]

In [9]: id(f1.bar)
Out[9]: 39526344

In [10]: id(f2.bar)
Out[10]: 39526344