Pythonic way to do static local variables?

[Lonnie Princehouse]

A quick, hackish way to keep a static variable is to declare it as a
parameter and give it a default value.  The parameter list is evaluated
when the function is compiled, not when it is called.   The underscores
are added as per convention to indicate that the variable is
special/private.

Example-

def cumulative_sum(arg, __static__ = []):
    __static__.append(arg)
    return reduce(lambda a,b: a + b, __static__)

#-------------------

>>> cumulative_sum(1)
1
>>> cumulative_sum(1)
2
>>> cumulative_sum(1)
3