python Be careful with mutable default arguments

Be careful with mutable default arguments

>>> def foo(x=[]):
...     x.append(5)
...     print x
...
>>> foo()
[5]
>>> foo()
[5, 5]
>>> foo()
[5, 5, 5]

Instead, you should use.

>>> def foo(x=None):
...     if x is None:
...         x = []
...     x.append(5)
...     print x
>>> foo()
[5]
>>> foo()
[5]