[Python] Traceback (Most recent call last) error in python

def adder(l1,l2,op):

if(op == 0):

carry = 0
result = []
l1.reverse()
l2.reverse()

for x in range(0, 4):

sum = l1[x] + l2[x] + carry

if(sum == 0):
result[x] = 0
carry = 0
elif(sum == 1):
result[x] = 1
carry = 0
elif(sum == 2):
result[x] = 0
carry = 1
elif(sum == 3):
result[x] = 1
carry = 1

if(x == 2):
carry_in = carry
if(x == 3):
carry_out = carry

result.reverse()

overflow = carry_in ^ carry_out
sign = result[3]
zero = not(result[0] | result[1] | result[2] | result[3])
sign_of_true_result = overflow ^ sign

print result,carry,overflow,sign,zero,sign_of_true_result


number1 = []
number2 = []
print "Enter the bits of number 1 one by one: "

x = 0
while(x < 4):

digit_1 = raw_input()

if(digit_1 != '0' and digit_1 != '1'):
print "Please enter either 0 or 1"
continue
else:
x = x + 1
number1.append(int(digit_1))

print "Enter the bits of number 2 one by one: "

y = 0
while(y < 4):

digit_2 = raw_input()

if(digit_2 != '0' and digit_2 != '1'):
print "Please enter either 0 or 1"
continue
else:
y = y + 1
number2.append(int(digit_2))

op = int(raw_input("Press 0 for addition or 1 for substraction (Op): "))

if __name__ == '__main__':
adder(number1,number2,op)


I am trying to implement binary adder for 4 bits. I get the following error. What's the problem with line 22 ? I don't understand why there is an out of range error.

Error:

Traceback (most recent call last):
File "ex9.py", line 85, in <module>
adder(number1,number2,op)
File "ex9.py", line 22, in adder
result[x] = 0
IndexError: list assignment index out of range

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