Detailed explanation of Python global variable global keyword

Variable scope in Python

1 Generally, variables defined outside the function become global variables, and variables defined inside the function are called local variables.

Global variables are available in all scopes, and local variables are available only in this function. The order of using variables is local variables > Global variables, that is, local variables are preferred

Then the question arises, what should I do if I want to use a global variable in a function or change the value of a global variable?

Python Global Variable global Keyword

In order to solve the problem of using global variables in functions, python adds global keyword, which can specify the scope of variables by using its characteristics.

Python Global Variable global Keyword: Declare variable var to be global

Code instance

Example 1:

Function preferences to local variables

str = 'global'
def func1():
  str = 'local'
  print(str)
func1()
print(str)

Results:

local

global

Example 2:

Use global variables without local variables

str = 'global'
def func1():
  print(str)
func1()
print(str)

Results:

global

global

Example 3:

Change the value of the global variable. As you can see from Example 1, the value of the global variable cannot be changed by the assignment within the function, so the global keyword is needed

str = 'global'
def func1():
  global str
  str = 'local'
  print(str)
func1()
print(str)

Results:

local

local

Other usages

You can specify multiple global variables using the same global statement.

For example

global var1, var2, var3

Special type

String and numeric types cannot be modified locally unless global keywords are used, but lists and dictionaries can be modified but cannot be re-assigned. If re-assignment is needed, global variables need to be defined using global inside the function

Code Example 1:

list = ['global', 'ofstack.com']
def func1():
  list.append('www.ofstack.com')
  print(list)
func1()
print(list)

Results:

['global', 'ofstack.com', 'www.ofstack.com']

['global', 'ofstack.com', 'www.ofstack.com']

It is found that the above list does not use global, but the value has changed, indicating that the list can be modified locally

Code Example 2:

list = ['global', 'ofstack.com']
def func1():
  list = ['docs.ofstack.com']
  print(list)
func1()
print(list)

Results:

['docs.ofstack.com']

['global', 'ofstack.com']

Local variable assignment cannot change the value of global variable

Code Example 3:

list = ['global', 'ofstack.com']
def func1():
  global list
  list = 'docs.ofstack.com'
  print(list)
func1()
print(list)

Results:

docs.ofstack.com

docs.ofstack.com

After using the global keyword, the variable is reassigned

This article mainly explains the use of Python global variable global keyword. For more information about the use of Python global variable, please see the following related links