initialization in argument definitions

[Brentt]

Hi, I know this is a terribly simple question, but the docs seem to be
designed for people who probably find a the answer to this question
terribly obvious. But its not at all obvious to me.

I can't figure out why when I define a function, a variable
(specifically a list) that I define and initialize in the argument
definitions, will not initialize itself every time its called. So for
example, when making a simple list of a counting sequence from num (a
range list), if I call the function multiple times, it appends the
elements to the list generated the times it was called before, even
though the variable for the list is initialized in the argument
definitions.

def foo_range(num,aList = []):
aList = []
#why is this seemingly extra initialization necessary? shouldn't it be
initialized in the argument definitions?
#but if its not there and the function is called multiple times the
elements generated (see below)
#append to the list generated before.
while num <= 10:
aList.append(num)
num +=1
else:
return aList

Why is this? Thanks, hope its not a stupid quesiton.