Global variable declaration in python 3

Question

I am trying to understand how global variables impact. Consider the below code as example

person = "Dave"
def display(person):
    global i 
    i = "Jack"
    print("here is ",person) 
display(person) 
display(i)

This results to:
here is Dave
here is Jack

I need to understand how "here is Jack" is printed

The below code results in error that i is not defined.

person = "Dave"
def display(person):
    global i 
    i = "Jack"
    print("here is ",person) 
display(i)

I don't understand this as well.

Please favour.

Answer 1

@Gayathri, the first thing is that global keyword is used to refer the global variables declared in the program or to declare any variable as global which is going to appear inside function block (as in your case).

Let you please understand the difference between following 2 code samples.

» Using global keyword:

i = 10;

def func():
    global i;
    i = 20; # Modifying global i
    print(i); # 20

print(i); # 10
func();
print(i); # 20

» Without using global keyword:

i = 10;
def func():
    i = 20; # It won't modify global i, here i is local to func()
    print(i); # 20

print(i); # 10
func(); 
print(i); # 10

Now, let's focus on the main problem.

✓ In first case, the value of local person in def display(person): is "Dave" and it is printing here is Dave & after that it is creating global i and setting its value to 'Jack'. In second call i.e. display(i) is passing the set value Jack of i which is assigned to local person variable available in def display(person): and therefore it is printing here is Jack and no error.

✓ In second case, no explicit assignment or function call for setting value of i before function call display(i) therefore there's an error.

Answer 2

On the second example, when you call display, i is not defined yet, but in the first example, as you had called display before, i is defined and have a value. If you change the order in the first example, it won't work also.

i is only defined after a display has been called.