代码1：
# coding:utf-8
sum = 5
def add(x, y):
print sum
sum = x + y
if __name__ == '__main__':
add(7, 8)
当我运行上面的代码时，出现以下错误：
ssspure:python ssspure$ python test.py
Traceback (most recent call last):
File "test.py", line 11, in <module>
add(7, 8)
File "test.py", line 6, in add
print sum
UnboundLocalError: local variable 'sum' referenced before assignment
代码2：
ssspure:python ssspure$ python test.py
Traceback (most recent call last):
File "test.py", line 11, in <module>
add(7, 8)
File "test.py", line 6, in add
print …