[Python] Calculating Minimum Edit Distance for unequal strings python

I am trying to implement Minimum Edit Distance with the substitution cost of 2. Following is the code I've so far. It works well for strings of equal length but generates error for the unequal strings. Kindly correct me where i am wrong

def med(source, target):
# if len(x) > len(y):
# print("insode if")
# source, target = y, x
print(len(source), len(target))
cost = [[0 for inner in range(len(source)+1)] for outer in
range(len(target)+1)]

global backtrace
backtrace = [[0 for inner in range(len(source)+1)] for outer in
range(len(target)+1)]
global SUB
global INS
global DEL

for i in range(0,len(target)+1):
cost[0] = i

for j in range(0,len(source)+1):
cost[0][j] = j

for i in range(1,len(target)+1):
for j in range(1,len(source)+1):
if source[i-1]==target[j-1]:
cost[j] = cost[i-1][j-1]
else:
deletion = cost[i-1][j]+1
insertion = cost[j-1]+1
substitution = cost[i-1][j-1]+2
cost[j] = min(insertion,deletion,substitution)

if cost[j] == substitution:
backtrace[j] = SUB
elif cost[j] == insertion:
backtrace[j] = INS
else:
backtrace[j] = DEL


return cost[j]

med("levenshtein","levels")


The error i get is:

---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-26-86bf20ea27c7> in <module>()
49 return cost[j]
50
---> 51 med("levenshtein","levels")

<ipython-input-26-86bf20ea27c7> in med(source, target)
31 for i in range(1,len(target)+1):
32 for j in range(1,len(source)+1):
---> 33 if source[i-1]==target[j-1]:
34 cost[j] = cost[i-1][j-1]
35 else:

IndexError: string index out of range

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