How do I calculate a mean with python?

[William Bryant]

Hey I am new to python so go easy, but I wanted to know how to make a program 
that calculates the maen.

List = [15, 6, 6, 7, 8, 9, 40]
def mean():
    global themean, thesum
    for i in List:
        thecount = List.count(i)
        thesum = sum(List)
    themean = thesum / thecount

Why doesn't this work?
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