Python图灵机脚本报错：已赋值变量仍提示未定义（UnboundLocalError）

核心原因 #

Python中，当函数内部存在对变量的赋值操作时，解释器会默认将该变量视为局部变量，即使全局作用域已定义同名变量。你的read()函数里有state = 2这类赋值语句，所以执行if state == 0时，解释器认为state是局部变量，但此时还未赋值，从而抛出错误。同理index变量也存在相同问题。

修复步骤 #

1. 声明全局变量 #

在涉及修改全局变量的函数开头，添加global声明，告诉解释器使用全局作用域的变量：

def MoveRight():
    global index
    index += 1

def MoveLeft():
    global index
    index -= 1

def read():
    global state, index, tape
    # 原函数逻辑不变

2. 修复未调用的函数 #

代码中有几处仅写了MoveRight或MoveLeft，未加括号调用，导致函数不会执行，需补充括号：

• 将MoveRight改为MoveRight()
• 将MoveLeft改为MoveLeft()

3. 优化逻辑（可选） #

原read()函数存在重复的状态判断（如两次if state ==4），可能导致逻辑冲突，建议改用elif来确保同一状态下仅触发一个分支：

def read():
    global state, index, tape
    if state == 0:
        if tape[index] == 0:
            tape.insert(index, "1")
            state = 2
            MoveRight()
            read()
    elif state == 2:
        if tape[index] == 0:
            tape.insert(index, "1")
            state = 3
            MoveRight()
            read()
    elif state == 3:
        if tape[index] == 0:
            tape.insert(index, "1")
            state = 4
            MoveLeft()
            read()
    elif state == 4:
        if tape[index] == 1:
            tape.insert(index, "0")
            state = 4
            MoveLeft()
            read()
        elif tape[index] == 0:
            tape.insert(index, "0")
            state = 5
            MoveRight()
            read()
    elif state == 5:
        if tape[index] == 0:
            tape.insert(index, "0")
            state = 5
            MoveRight()
            read()
        elif tape[index] == 1:
            tape.insert(index, "1")
            state = 0
            MoveRight()
            read()

修复后的完整代码 #

#Turing Machine In Python
#by adrian wheeler

#the script

index = 5
state = 0
tape = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]


def MoveRight():
    global index
    index += 1

def MoveLeft():
    global index
    index -= 1

def read():
    global state, index, tape
    if state == 0:
        if tape[index] == 0:
            tape.insert(index, "1")
            state = 2
            MoveRight()
            read()
    elif state == 2:
        if tape[index] == 0:
            tape.insert(index, "1")
            state = 3
            MoveRight()
            read()
    elif state == 3:
        if tape[index] == 0:
            tape.insert(index, "1")
            state = 4
            MoveLeft()
            read()
    elif state == 4:
        if tape[index] == 1:
            tape.insert(index, "0")
            state = 4
            MoveLeft()
            read()
        elif tape[index] == 0:
            tape.insert(index, "0")
            state = 5
            MoveRight()
            read()
    elif state == 5:
        if tape[index] == 0:
            tape.insert(index, "0")
            state = 5
            MoveRight()
            read()
        elif tape[index] == 1:
            tape.insert(index, "1")
            state = 0
            MoveRight()
            read()
    
#running the script

read()

内容的提问来源于stack exchange，提问作者Adrian Wheeler