Python UnboundLocalError: local variable 'xxx' referenced before assignment

全局变量 与 局部变量

定义了一个全局变量
​​​arr = []​​

在方法内使用时：

def main():
    if len(arr) == 0:
        f = open('log.txt','r')
        arr = list(f.readlines())
        f.close()

这样报错：

Traceback (most recent call last):
  File "select.py", line 272, in <module>
    main()
  File "select.py", line 33, in main
    if len(arr) == 0:
UnboundLocalError: local variable 'arr' referenced before assignment

---

改一下：

def main():
    if len(arr) == 0:
        global arr
        f = open('log.txt','r')
        arr = list(f.readlines())
        f.close()

结果还是报错：

  File "select.py", line 34
    global arr
    ^
SyntaxError: name 'arr' is used prior to global declaration

---

继续改：

def main():
    global arr
    if len(arr) == 0:
        f = open('log.txt','r')
        arr = list(f.readlines())
        f.close()

终于OK了

---