local variable 'a' referenced before assignment

代码如下：
def a():
aa = '123'
return aa

def main():
a = a()
print a

main()
>>>UnboundLocalError: local variable 'a' referenced before assignment

这样的话就没问题：
def a():
aa = '123'
return aa

def main():
# 变量不是a就行
a2 = a()
print a2

main()
>>>'123'