Python中global和nolocal作用域的理解
原創:https://www.cnblogs.com/z360519549/p/5172020.html
python引用變數的順序: 當前作用域區域性變數-> 閉包函式外的函式中變數->當前模組中的全域性變數->python內建變數 。
一 、global
global關鍵字用來在函式或其他區域性作用域中使用全域性變數。但是如果不修改全域性變數也可以不使用global關鍵字。
gcount = 0
def global_test():
gcount+=1
print (gcount)
global_test()
D:\Python34\python.exe E:/PycharmProjects/Day3/globaltest.py
Traceback (most recent call last):
File “E:/PycharmProjects/Day3/globaltest.py”, line 6, in
global_test()
File “E:/PycharmProjects/Day3/globaltest.py”, line 4, in global_test
gcount+=1
UnboundLocalError: local variable ‘gcount’ referenced before assignment
Process finished with exit code 1
第一行定義了一個全域性變數,(可以省略global關鍵字)。
在global_test 函式中程式會因為“如果內部函式有引用外部函式的同名變數或者全域性變數,並且對這個變數有修改.那麼python會認為它是一個區域性變數,又因為函式中沒有gcount的定義和賦值,所以報錯。
二、宣告全域性變數,如果在區域性要對全域性變數修改,需要在區域性也要先宣告該全域性變數:
gcount = 0
def global_test():
global gcount
gcount+=1
print (gcount)
global_test()
如果在函式中宣告 gcount 是全域性變數,即可對其進行修改。 正確輸出 1 。
三、 在區域性如果不宣告全域性變數,並且不修改全域性變數。則可以正常使用全域性變數:
gcount = 0
def global_test():
print (gcount)
global_test()
如果在區域性不修改全域性變數,程式正確輸出 0 。
四、nonlocal關鍵字用來在函式或其他作用域中使用外層(非全域性)變數。
def make_counter():
count = 0
def counter():
nonlocal count
count += 1
return count
return counter
def make_counter_test():
mc = make_counter()
print(mc())
print(mc())
print(mc())
make_counter_test()
'''
輸出:
1
2
3
'''
五、例項
1、
def scope_test():
def do_local():
spam = "local spam" #此函式定義了另外的一個spam字串變數,並且生命週期只在此函式內。此處的spam和外層的spam是兩個變數,如果寫出spam = spam + “local spam” 會報錯
def do_nonlocal():
nonlocal spam #使用外層的spam變數
spam = "nonlocal spam"
def do_global():
global spam
spam = "global spam"
spam = "test spam"
do_local()
print("After local assignmane:", spam)
do_nonlocal()
print("After nonlocal assignment:",spam)
do_global()
print("After global assignment:",spam)
scope_test()
print("In global scope:",spam)
'''
輸出是:
After local assignmane: test spam
After nonlocal assignment: nonlocal spam
After global assignment: nonlocal spam
In global scope: global spam
'''
2、
def add_b():
global b
b = 42
def do_global():
global b
b = b + 10
print(b)
do_global()
print(b)
add_b()
print(b)
'''
以上程式碼輸出:
52
52
52
'''
def add_b():
global b
b = 42
def do_global():
#global b
b = b + 10
print(b)
do_global()
print(b)
add_b()
'''
以上程式碼報錯:
Traceback (most recent call last):
File "E:/PycharmProjects/OOP/exe1.py", line 42, in <module>
add_b()
File "E:/PycharmProjects/OOP/exe1.py", line 40, in add_b
do_global()
File "E:/PycharmProjects/OOP/exe1.py", line 38, in do_global
b = b + 10
UnboundLocalError: local variable 'b' referenced before assignment
原因: global 定義的 b ,只能引用,不能修改。
'''
def add_b():
global b
b = 42
def do_global():
global a
a = b + 10
print(b)
do_global()
print(a)
add_b()
print("a = %s , b = %s " %(a, b))
'''
輸出:
42
52
a = 52 , b = 42
'''
def add_b():
#global b
b = 42
def do_global():
global b
b = 10
print(b)
do_global()
print(b)
add_b()
print(" b = %s " % b)
'''
以上程式碼輸出:
10
42
b = 10
'''
def add_b():
#global b
b = 42
def do_global():
nonlocal b
b = 10
print(b)
do_global()
print(b)
add_b()
'''
以上程式碼輸出:
10
10
'''
def add_b():
#global b
b = 42
def do_global():
nonlocal b
b = 10
print(b)
do_global()
print(b)
add_b()
print(" b = %s " % b)
'''
以上程式碼報錯:
print(" b = %s " % b)
NameError: name 'b' is not defined
說明: nonlocal 適用於在區域性函式 中 的區域性函式, 把最內層的區域性 變數設定成外層區域性可用,但是還不是全域性的。
'''
def add_b():
#global b
#b = 42
def do_global():
nonlocal b
b = 10
print(b)
do_global()
#print(b)
add_b()
'''
以上程式碼報錯:
File "E:/PycharmProjects/OOP/exe1.py", line 37
nonlocal b
SyntaxError: no binding for nonlocal 'b' found
nonlocal 要繫結一個區域性變數。
'''
def add_b():
#global b
#b = 42
def do_global():
global b
b = 10
print(b)
do_global()
print(b)
add_b()
print(" b = %s " % b)
'''
以上程式碼輸出:
10
10
b = 10
'''
def add_b():
#global b
#b = 42
def do_global():
global b
b = 10
print(b)
do_global()
#b = b + 20
print(b)
add_b()
b = b + 30
print(" b = %s " % b)
'''
以上程式碼輸出:
10
10
b = 40
'''
def add_b():
#global b
#b = 42
def do_global():
global b
b = 10
print(b)
do_global()
b = b + 20
print(b)
add_b()
b = b + 30
print(" b = %s " % b)
'''
以上程式碼報錯:
b = b + 20
UnboundLocalError: local variable 'b' referenced before assignment
修改如下:
'''
def add_b():
#global b
b = 42
def do_global():
global b
b = 10
print(b)
do_global()
b = b + 5
print(b)
add_b()
b = b + 30
print(" b = %s " % b)
'''
以上程式碼輸出:
10
47
b = 40
'''