基本反编译器中的Python“分配前已引用本地变量'pc'”问题

Question

A friend and myself are working on creating a basic proof-of-concept decompiler that takes a string of hex values and returns a more readable version. Our code is listed below

testProgram = "00 00 FF 55 47 00"
# should look like this
# NOP
# NOP
# MOV 55 47
# NOP

pc = 0
output = ""

def byte(int):
    return testProgram[3 * int:3 * int + 2]

def interpret():

    currentByte = byte(pc)

    if currentByte == "00":
        pc += 1
        return "NOP"

    if currentByte == "FF":
        returner = "MOV " + byte(pc + 1) + " " + byte(pc + 2)
        pc += 3
        return returner

while(byte(pc) != ""):
    output += interpret() + "\n"

print(output)

however, running the code tells us this

Traceback (most recent call last):
  File "BasicTest.py", line 62, in <module>
    output += interpret() + "\n"
  File "BasicTest.py", line 50, in interpret
    currentByte = byte(pc)
UnboundLocalError: local variable 'pc' referenced before assignment

Because pc is a global variable, shouldn't it be usable from anywhere? Any and all help is appreciate - if you spot other errors, feel free to leave a comment pointing them out!

Answer 1

Been seeing this a lot lately. When you do

if currentByte == "00":
    pc += 1  # <----------
    return "NOP"

You're assigning to the local variable pc , but pc isn't declared yet in the local scope. If you want to modify the global pc you need to declare that explicitly at the top of the function

global pc