# Be careful using mutable data structures in method definitions.
#
# The assignment of the "foo" argument's default value is made only the
# first time the function is actually evaluated on the runtime.
#
# Then it uses the same memory address for the default argument value,
# sharing it with all instances of that class.

class Foo(object):
    def __init__(self, foo=[]):
        foo.append(1)
        self.foo = foo


Foo().foo #=> [1]
Foo().foo #=> [1, 1]