Add checksum to string based on ISBN method

stringChecksum.py

#!/usr/bin/env python
sku = "ABCDEFG"

def checksum(sku):
    sum = 0
    for i,c in enumerate(sku):
        o = ord(c)
        #print("{},{}".format(c,o))
        if i % 2 == 1:
            sum += o
        else:
            sum += o * 3

    letters = [x for x in "ABCDEFGHIJ"]
    checksum = letters[sum%10]
    return(checksum)

def verify(sku):
    sku = [x for x in sku]
    check = sku[len(sku)-1]
    sku = sku[0:(len(sku)-1)]

    if check == checksum(sku):
        return(True)
    return(False)

joint = sku + checksum(sku)
print(sku)
print(joint)
print(verify(joint))

Seems like a handy addition for hand entered SKU etc