The difference between global variables and local variables in Python (detailed code explanation)

#Global variables are variables defined and declared outside the function and we need to use them inside the function.

#这个函数使用全局变量s
def f():  
    print s  
  
# 全局作用域
s = "I love Python"
f()

Output:

I love Python

If a variable with the same name is defined within a function scope, then it will only print the value given within the function instead of the global value.

# 这个函数有一个与s同名的变量。
def f():  
    s = "Me too."
    print s  
  
# 全局作用域
s = "I love Python" 
f() 
print s

Output:

Me too
I love Python

Before we call the function f(), the variable s is defined as the string "I love Python". The only statement in f() is the "print s" statement. Since there is no local s, the value of global s will be used.

The question is, what happens if we change the value of s inside function f()? Will it affect the overall situation?

We test it in the following code:

def f():  
    print s 
  
    # 如果我们在下面评论，这个程序不会显示错误。
    s = "Me too."
  
    print s 
  
#全局作用域
s = "I love Python" 
f() 
print s

Output:

Line 2: undefined: Error: local variable 's' referenced before assignment

In order for the above program to work, we need to use the "global" keyword. If we want to assign/change them, we just need to use the global keyword in the function. Printing and accessing do not require globals.

Python "assumes" we want a local variable, so the first print statement throws this error message because of the assignment inside f(). Any variable changed or created inside a function is local if it has not been declared as a global variable. To tell Python that we want to use global variables, we must use the keyword "global"

As shown in the following example:

# 这个函数修改全局变量's' 
def f(): 
    global s 
    print s 
    s = "Look for Python"
    print s  
  
#全局作用域
s = "Python is great!" 
f() 
print s

Output:

Python is great!
Look for Python.
Look for Python.

A nice Example:

a = 1
  
# 使用global，因为没有本地'a' 
def f(): 
    print 'Inside f() : ', a 
  
#变量“a”被重新定义为局部变量
def g():     
    a = 2
    print 'Inside g() : ',a 
  
#使用全局关键字修改全局'a'
def h():     
    global a 
    a = 3
    print 'Inside h() : ',a 
  
# 全局作用域
print 'global : ',a 
f() 
print 'global : ',a 
g() 
print 'global : ',a 
h() 
print 'global : ',a

Output:

global :  1
Inside f() :  1
global :  1
Inside g() :  2
global :  1
Inside h() :  3
global :  3

Related recommendations: "Python Tutorial"

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