Understand Python’s global variables and local variables

Understanding Python's global variables and local variables

1. If the variable name inside the defined function appears for the first time and precedes the = symbol, it can be considered to be defined as a local variable. In this case, regardless of whether the variable name is used in the global variable, the local variable is used in the function. For example:

# _*_ coding: utf-8 _*_
num = 110
def func():
    num = 1
    print(num)
func()
输出结果：1

# _*_ coding: utf-8 _*_
num = 110
def func():
    num += 1
    print(num)
func()
输出结果：

UnboundLocalError: local variable 'num' referenced before assignment

Error message: The local variable num is applied before assignment, that is, it is used without defining the variable, thus again It proves that a local variable is defined here instead of using the global num.

Summary: If the variable name inside the function appears for the first time and appears before =, it is regarded as defining a local variable.

2. If the variable name inside the function appears for the first time and appears after =, and the variable has been defined in the global domain, the global variable will be referenced here. If the variable does not exist in the global domain Definition, of course there will be a "variable is not defined" error. For example:

# _*_ coding: utf-8 _*_
num = 110
def func():
    num1 = num +  1
    print(num1)
func()
输出结果：
111

3、函数中使用某个变量时，该变量名既有全局变量也有同名的局部变量，则会使用局部变量，例如：

# _*_ coding: utf-8 _*_
num = 110
def func():
    num = 200
    num1 = num +  1
    print(num1)
func()
输出结果：
201

4、在函数中，如果想给全局变量赋值，则需要用关键字global声明，例如：

# _*_ coding: utf-8 _*_
num = 100
def func():
    num = 300
    num1 = num +  1
    print(num1)
func()
print num
输出结果：

301<br>100

declare num:

# _*_ coding: utf-8 _*_
num = 100
def func():
    global  num
    num = 300
    num1 = num +  1
    print(num1)
func()
print num

输出结果：

301<br>300

from：

<br>

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