Its all about the scope of a variable 

variable defined at the top of all functions will have global scope.

variables defined in function are called local variables & the life of that variable is  with in the function only .

#### Global variable
a = 10
#### funtion defination
def display():
b=20
print(“the value of a from a display function”,a)
print(“the value of b from a display function”,b)

###### function call
display()

print(“the value of b from main function”,b)

o/p

the value of a from a display function 10
the value of b from a display function 20

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-4-bfab6779e705> in <module>
      9 display()
     10 
---> 11 print("the value of b from a display function",b)

NameError: name 'b' is not defined

Calling local variable that is present in another function will fail

#### Global variable
a = 10
#### funtion defination
def display():
b=20
print(“the value of a from a display function”,a)
print(“the value of b from a display function”,b)
###### function call
display()

def display2():
print(“the value of b in second function”,b)
display2()

o/p:

the value of a from a display function 10
the value of b from a display function 20

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-2-a8be151a7df3> in <module>
     12     print("the value of b in second function",b)
     13 
---> 14 display2()

<ipython-input-2-a8be151a7df3> in display2()
     10 
     11 def display2():
---> 12     print("the value of b in second function",b)
     13 
     14 display2()

NameError: name 'b' is not defined