What value should be passed to make a function use the default argument value?

[LaundroMat]

Suppose I have this function:

def f(var=1):
return var*2

What value do I have to pass to f() if I want it to evaluate var to 1?
I know that f() will return 2, but what if I absolutely want to pass a
value to f()? "None" doesn't seem to work..

Thanks in advance.