Python to increment and decrement a binary string without converting to int. Decr does not take into account underflows

incr_decr_binary_string.py

def incr(bit_string):
    carry = False
    bits = list(bit_string[::-1])
    for i, bit in enumerate(bits):
        carry = bit != "0"
        if bit == "0":
            bits[i] = "1"
            break
        else:
            bits[i] = "0"
    if carry:
        bits.append("1")
    return "".join(bits[::-1])


def decr(bit_string):
    bits = list(bit_string[::-1])
    for i, bit in enumerate(bits):
        if bit == "1":
            bits[i] = "0"
            break
        else:
            bits[i] = "1"
    return "".join(bits[::-1])

print incr("101")
print incr("1111")
print decr("101")
print decr("110")