Hex to MIPS bitwise

#!/usr/bin/python
#coding=utf-8
def get_cod():
    return int(raw_input('Qual a instrução em código de máquina? '), 16)

def decode(b):
    def get_opcode(opcode):
        opcodes = {
            8 : "addi",
            12 : "andi",
            4 : "beq",
            5: "bne",
            35 : "lw",
            13 : "ori",
            10 : "slti",
            43 : "sw"
        }
        if opcode in opcodes.keys():
            return opcodes[opcode]
        raise LookupError

    def get_funct(funct):
        functs = {
            32 : "add",
            36 : "and",
            39 : "nor",
            37 : "or",
            42 : "slt",
            0 : "sll",
            2 : "srl",
            34 : "sub"
        }
        if funct in functs.keys():
            return functs[funct]
        raise LookupError

    def get_reg(reg):
        if not(reg in (24, 25)):
            return ("$zero", "$at", "$v1", "$v2", "$a0", "$a1", "$a2", "$a3", "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7", "$s0", "$s1", "$s2", "$s3", "$s4", "$s5", "$s6", "$s7", None, None, "$k0", "$k1", "$gp", "$sp", "$fp", "$ra")[reg]
        raise LookupError

    mips = ()
    opcode = (b >> 26) & 63
    #R
    if opcode == 0:
        mips = (get_funct(b & 63), (get_reg((b >> 21) & 31), get_reg((b >> 16) & 31), get_reg((b >> 11) & 31), str((b >> 6) & 31)))
    #J
    elif opcode == 2:
        mips = ("j", (str(b & 67108863)))
    #I
    else:
        mips = (get_opcode(opcode), (get_reg((b >> 21) & 31), get_reg((b >> 16) & 31), str(b & 65535)))
    return " ".join((mips[0], ", ".join(mips[1])))

while True:
    try:
        decode(get_cod())
    except LookupError:
        print "Houve um erro: a instrução não é válida."
    except:
        print "Houve um erro na execução do programa."
        break;