Trying to understand global variables

[357mag]

Just learning about using a global variable. I have this going:

x = 4

def show_value():
    print('Value of x: '+ str(x))
   # x += 3
   # print('Value of x: '+ str(x))
    
show_value()

I had to comment out those two lines in order to get the program to run. I thought that in my show_value function I could modify the value of the variable, but apparently I cannot because the compiler flagged it.

I don't quite get it. If I declare a global variable that can be seen and accessed in the show_value function, then why can't I modify it in the function as well?

[357mag]

Okay I found out why. You have to use the global keyword to tell the interpreter that the main function intends to modify the variable.

[deanhystad]

Not modify, assign.

In Python you create a variable when you do assignment. By default, the variable is created in the context where the assignment is made. If you use the global keyword, this tells Python to look in the global namespace/scope for the variable.

You do not need to use global when modifying a mutable object. For example:

x = [1]

def modify(value):
    x[0] = value

modify(5)
print(x)

Output:
5

This works because I am not assigning a value to x, I am modifying the object that was already assigned to x.

[357mag]

Not modify, assign.

In Python you create a variable when you do assignment. By default, the variable is created in the context where the assignment is made. If you use the global keyword, this tells Python to look in the global namespace/scope for the variable.

You do not need to use global when modifying a mutable object. For example:

x = [1]

def modify(value):
    x[0] = value

I don't really understand your code. What are the brackets for? It reminds me of a list or an array.

modify(5)
print(x)

Output:
5

This works because I am not assigning a value to x, I am modifying the object that was already assigned to x.

[357mag]

Why the brackets around the number?

[deanhystad]

This

x = [1]

says "Make a list that contains the number 1. Assign the list to x." The square brackets tell Python to create a list.

Lists are mutable objects, meaning they are something you can change. The modify function in my example uses indexing to change the list so it contains the number 5. You can also append to lists, remove items from lists, and make slices of the list. Lists are very useful in Python.