gistfile1.txt

class What(object):
    def __init__(self, arg={}):
        self.arg = arg
        assert not len(self.arg)
        arg[1] = 2

What()
# Whoops... arg is a mutable object
What()

"""
Traceback (most recent call last):
  File "a.py", line 9, in <module>
    What()
  File "a.py", line 5, in __init__
    assert not len(self.arg)
AssertionError
"""