關於python的變數作用域

#定義變數a
>>> a = 0
>>> print a
0
 
#定義函數p()
>>> def p():
...     print a
...     
>>> p()
0
 
#定義函數p2()
>>> def p2():
...     print a
...     a = 3
...     print a
...     
>>> p2() # 運行出錯，外部變數a先被引用，不能重新賦值
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
  File "<interactive input>", line 2, in p2
UnboundLocalError: local variable 'a' referenced before assignment
 
#定義函數p3()
>>> def p3():
...     a = 3 # 不引用直接賦值
...     print a
...     
>>> p3()
3
>>> print a
0 # 外部變數a並未改變