python并在if语句中分配变量值

Question

I'm somewhat of a beginner in python and I swear I got a very similar program to what I'm doing right now to work. But, for some reason, I can't make it work. I was able to pinpoint my problem and created a fake program to play around with it. Here is what the program:

global heading
global heading2
global a

heading=2
a=2
heading2=4

def function ():
    if a==2:
        heading=heading2
        print 'yes'
        print heading

function()       
print heading

This is what appears:

yes
4
2

Why doesn't the heading variable heading keep the value 4? I tried putting return heading all over. Didn't work. I tried putting the variables in the parentheses of the function. Didn't work either... Do you know what I'm doing wrong?

Answer 1

global statement is meaningless outside of a function. If you want to modify the global variable, instead of introducing a local one, you need to put global inside the function

def foo():
    global x
    x = x2

Also, don't use globals.

Answer 2

The line:

    heading=heading2

Creates a new local variable called heading , which is different to the other varaible called heading that you passed into the function.

You can make the function assign to external variables by adding:

global heading

before you assign to heading:

def function():
    global heading
    if a==2:
        heading=heading2
        print 'yes'
        print heading

Answer 3

Inside function, you are creating a local variable called heading . This isn't the same heading declared outside of the function.