python - 如何在 Python 中使用全局变量？

Question

我不确定为什么当我声明一个全局变量时它不起作用...

first_read = True

def main():

    if (first_read == True):
        print "hello world"
        first_read = False

    print 'outside of if statement'

if __name__ == '__main__':
    main()

我的回溯显示以下错误:

Traceback (most recent call last):
   File "true.py", line 12, in <module>
      main()   
   File "true.py", line 5, in main
     if (first_read == True): 
UnboundLocalError: local variable 'first_read' referenced before assignment

Answer 1

您必须将变量定义为全局变量:

first_read = True

def main():
    global first_read
    if (first_read == True):
       print "hello world"
       first_read = False

    print 'outside of if statement'

if __name__ == '__main__':
    main()