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```#This code is GPL3

def calc_check_digit(value):
"""calculate check digit, they are the same for both UPCA and UPCE"""
check_digit=0
odd_pos=True
for char in str(value)[::-1]:
if odd_pos:
check_digit+=int(char)*3
else:
check_digit+=int(char)
odd_pos=not odd_pos #alternate
check_digit=check_digit % 10
check_digit=10-check_digit
check_digit=check_digit % 10
return check_digit

def convert_UPCE_to_UPCA(upce_value):
"""Test value 04182635 -> 041800000265"""
if len(upce_value)==6:
middle_digits=upce_value #assume we're getting just middle 6 digits
elif len(upce_value)==7:
#truncate last digit, assume it is just check digit
middle_digits=upce_value[:6]
elif len(upce_value)==8:
#truncate first and last digit,
#assume first digit is number system digit
#last digit is check digit
middle_digits=upce_value[1:7]
else:
return False
d1,d2,d3,d4,d5,d6=list(middle_digits)
if d6 in ["0","1","2"]:
mfrnum=d1+d2+d6+"00"
itemnum="00"+d3+d4+d5
elif d6=="3":
mfrnum=d1+d2+d3+"00"
itemnum="000"+d4+d5
elif d6=="4":
mfrnum=d1+d2+d3+d4+"0"
itemnum="0000"+d5
else:
mfrnum=d1+d2+d3+d4+d5
itemnum="0000"+d6
newmsg="0"+mfrnum+itemnum
#calculate check digit, they are the same for both UPCA and UPCE
check_digit=calc_check_digit(newmsg)
return newmsg+str(check_digit)

#Usage example
print convert_UPCE_to_UPCA(UPCE)
```