When running my function my if statement is not running

I am trying to make a simple bubble sort, and the if statement i'm using to sort the numbers in my array is not running. Can anybody help me get this to run?

Here is my code:

def Bubble( a ):
    Flag = False
    while not Flag:
        Flag = True
        for i in range(0, len(a), -1):
            if a[i] > a[i+1]: #this if statement isn't running
                a[i], a[i + 1] = a[i + 1], a[i]
                print("hi")
                Flag = False



def main():
    a = GRN(10)
    acopy = a[:]
    Bubble(a)
    acopy.sort()
    print(a==acopy)
    print(a)
    print(acopy)


main()