我在python中更改/更新全局变量时遇到问题

Question

So I was having trouble understanding the basics of global variable as I am a beginner in python. I wanted to try to change a global variable but unfortunate it didn't work. Could any of you explain and help my fix my problem. Thanks!!

global x
x = 10
def NEWX():
     print (x)
     x = x + 5
     print (x)
print (x)
NEWX()
print(x)

#this displays 10 10 15 10

Answer 1

Try this refactor:

x = 10


def new_x():
    global x
    print(x)
    x = x + 5
    print (x)


print(x)
new_x()
print(x)

# prints 10 10 15 15

I was unable to run your original code, it complained about x being undefined in the function. Within the scope of the function, x is unknown. It's known at the higher scope, but not in your function. That's why the global x statement needs to be in the function - to let python know to use the x from the outer scope.

Answer 2

x = 10

def NEWX():
     global x
     print (x)
     x = x + 5
     print (x)

print (x)
NEWX()
print(x)

Global variable rules:

• global keyword outside a function has no effect
• Variable outside a function is global by default
• global keyword is needed to read and write global variable within a function

Answer 3

In your code, you are first printing the value of x. The function NEWX(): is then called which prints x, then adds 5 onto x and prints the new value. After the function is called, x is printed again.

To understand what is happening, check out this example that contains the fix:

 x = 10

def NEWX():
    print (x)
    global x
    x = x + 5
    print (x)
print (x)
NEWX()
print(x)