1. global
global keyword for using global variables or functions in other local scope. But if you do not modify global variables may not use the global keyword.
gcount = 0
def global_test():
gcount+=1
print (gcount)
global_test()
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Traceback (most recent call last):
File "D:\Desktop\test.py", line 6, in <module>
global_test()
File "D:\Desktop\test.py", line 4, in global_test
gcount+=1
UnboundLocalError: local variable 'gcount' referenced before assignment
The first line defines a global variable (global keyword may be omitted). In global_test program function because 如果内部函数有引用外部函数的同名变量或者全局变量,并且对这个变量有修改.那么python会认为它是一个局部变量,又因为函数中没有gcount的定义和赋值，所以报错.
If you want to modify the global variables in the local, local needs also first declare the global variable
gcount = 0
def global_test():
global gcount
gcount+=1
print (gcount)
global_test()
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1
[Finished in 0.1s]
If you do not declare local global variables, and does not modify global variables. Global Variables can be used normally (reference only not be modified)
gcount = 0
def global_test():
a = gcount + 1
print (gcount)
print(a)
global_test()
Output:
0
1
[Finished in 0.2s]
2. nonlocal
nonlocal keyword for the outer layer (non-global) variables in function or other scopes
count = 10 # 全局变量
def make_counter():
count = 0 # 局部变量， counter()函数中修改的这个count
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()
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1
2
3
The first call to make_countera function to modify the local variables count, each will modify call
3. Other examples enhance the understanding of the global and nonlocal
def scope_test():
def do_local():
spam = "local spam" #此函数定义了另外的一个spam字符串变量，并且生命周期只在此函数内。
# 此处的spam和外层的spam是两个变量，如果写出spam = spam + “local spam” 会报错
def do_nonlocal():
nonlocal spam #使用外层的spam变量
spam = "nonlocal spam"
def do_global():
global spam
spam = "global spam"
spam = "test spam" # 定义一个局部变量
do_local() # 该方法只定义了一个局部的spam, 并未对外边的spam修改
print("After local assignmane:", spam) # 因此此处输出的是test spam
do_nonlocal() # nonlocal申明了该函数中修改的是外层的spam
print("After nonlocal assignment:",spam) # 因此此处输出 nonlocal spam
do_global() # global 申明该函数修改的是全局变量
print("After global assignment:",spam) # 因此此处输出的依然是do_nonlocal函数修改后的spam
scope_test()
print("In global scope:",spam) # 输出全局变量spam, 即do_global函数中的spam
Output:
After local assignmane: test spam
After nonlocal assignment: nonlocal spam
After global assignment: nonlocal spam
In global scope: global spam
global defined variables b, can only be referenced within the function do_global within a function add_b, if you want to modify in the do_global, b must be declared global inside a function in do_global, indicating that global variables are modified outside b
def add_b():
global b
b = 42
def do_global():
global b
b = b + 10
print(b)
do_global()
print(b)
add_b()
Export
52
52
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))
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42
52
a = 52 , b = 42
def add_b():
b = 42
def do_global():
nonlocal b
b = 10
print(b)
do_global()
print(b)
add_b()
Export
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)
Export
10
10
Traceback (most recent call last):
File "D:\Desktop\test.py", line 11, in <module>
print(" b = %s " % b)
NameError: name 'b' is not defined
Description: nonlocal suitable for topical function in the local function, local variables in the innermost layer is arranged locally available, but not global
def add_b():
def do_global():
nonlocal b
b = 10
print(b)
do_global()
add_b()
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File "D:\Desktop\test.py", line 5
nonlocal b
^
SyntaxError: no binding for nonlocal 'b' found
Note: nonlocal To bind a local variable. But does not require global
def add_b():
def do_global():
global b
b = 10
print(b)
do_global()
print(b)
add_b()
b = b + 30
print(" b = %s " % b)
Export
10
10
b = 40
def add_b():
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)
Export
10
Traceback (most recent call last):
File "D:\Desktop\test.py", line 9, in <module>
add_b()
File "D:\Desktop\test.py", line 7, in add_b
b = b + 20
UnboundLocalError: local variable 'b' referenced before assignment
4. Reference
