UnboundLocalError（赋值前引用的局部变量）

Question

我正在尝试将脚本搜索跟踪编号并打印.txt文档的内容。使用此部件时出现此错误：

def lookup_trackingnum():
    logger.info('lookup_trackingnum started - B3                    ')
    print(' ')
    print(' ')
    print(' = = Look Up a Tracking Number = =  ')
    print(' ')
    print(' ')
    lookup_in = input('Tracking Number: ')
    try:
        lookup_path = 'lms19-files/exports/' + lookup_in + '.txt'
        logger.debug('lookup_path created - B3   -   BUG RESOLVING      ')
        with open(lookup_path, 'r') as lookup:
            logger.debug('lookup_path opened - B3   -   BUG RESOLVING       ')
            print(lookup.read())
            logger.debug('lookup_path printed - B3   -   BUG RESOLVING      ')
    except:
        print('ERROR: The Tracking Number ' + lookup_in + ' doesn\'t exist.')
        logger.error('Tracking Number doesn\'t exist - B2                ')
        print(' ')
        print(' ')
        print(' [E] Return to Dashboard  ')
        print(' ')
        logger.debug('Before lookupnav_in input - B3   -   BUG RESOLVING')
        lookupnav_in = input('Input: ')
        logger.debug('After lookupnav_in input - B3   -   BUG RESOLVING ')
    if lookupnav_in == 'E':
        logger.debug('If lookupnav_in - B3   -   BUG RESOLVING          ')
        startmenu()
        logger.debug('startmenu triggered - B3   -   BUG RESOLVING      ')
    else:
        print(lookupnav_in + ' isn\'t a valid input.')
        logger.error('Invalid Input - B3                                ')
        startmenu()

错误：

Traceback (most recent call last):
  File "/Users/User/PycharmProjects/lms19/lms19-beta.py", line 417, in <module>
    startmenu()
  File "/Users/User/PycharmProjects/lms19/lms19-beta.py", line 280, in startmenu
    lookup_trackingnum()
  File "/Users/User/PycharmProjects/lms19/lms19-beta.py", line 253, in lookup_trackingnum
    if lookupnav_in == 'E':
UnboundLocalError: local variable 'lookupnav_in' referenced before assignment

和日志数据：

lms19 : INFO - Logger Started - B6                                 [2019/04/11 15:20:11]
lms19 : CRITICAL - Setup skipped - B6                              [2019/04/11 15:20:11]
lms19 : INFO - Software Initialised - B6                           [2019/04/11 15:20:11]
lms19 : INFO - Main Script started - B7                            [2019/04/11 15:20:11]
lms19 : INFO - startmenu started - B4                              [2019/04/11 15:20:11]
lms19 : INFO - lookup_trackingnum started - B3                     [2019/04/11 15:20:13]
lms19 : DEBUG - lookup_path created - B3   -   BUG RESOLVING       [2019/04/11 15:20:15]
lms19 : DEBUG - lookup_path opened - B3   -   BUG RESOLVING        [2019/04/11 15:20:15]
lms19 : DEBUG - lookup_path printed - B3   -   BUG RESOLVING       [2019/04/11 15:20:15]

我尝试了在网上找到的多个东西（以及StackExchange），但我无法让它工作。

保罗先生，谢谢

Answer 1

lookupnav_in = input('Input: ')

这是您定义变量的地方。它位于try语句的except块中。

if lookupnav_in == 'E':

这是您第一次使用变量的地方。这就在except块之外。

除非实际发生异常，否则不执行except块。

所以想想如果try永远不会抛出异常将会发生什么。

永远不会定义变量。因此错误。

Answer 2

我认为这是因为您的变量lookupnav_in在您的Except块中声明，然后使用您的Except-block之外的if语句进行检查。如果未触发您的Except-block，则永远不会声明变量。这意味着在使用if语句检查之前，您必须在特定的Except块之外的其他地方声明它。