Variable is not defined Python

This is a simple program that adds two numbers but in two separate modules:

Main program :

import aritmetica as ar

print ("Introdusca 2 numeros para sumarlos ")

print ("Introdusca el primer numero : ")
num_a = int (input())
print ("")
print ("Introdusca el segundo numero : ")
num_b = int (input())
print ("")
suma_num = ar.suma(num_a,num_b)
print ("La suma total de los dos numeros es :")
print ("")
print (suma_num)

Arithmetic module :

def suma (num1,num2):

    return num1+num2

So far so good. The program adds the two numbers perfectly.

But I also want to experiment with a function without parameters. I have tried to do it in the following way:

import aritmetica as ar

print ("Introdusca 2 numeros para sumarlos ")

print ("Introdusca el primer numero : ")
num_a = int (input())
print ("")
print ("Introdusca el segundo numero : ")
num_b = int (input())
print ("")
suma_num = ar.suma()# elimino los parametros
print ("La suma total de los dos numeros es :")
print ("")
print (suma_num)

Arithmetic module :

def suma():

    numero = (num1 + num2)

    return numero

and I get the following error:

  

NameError: name 'num1' is not defined

What is the error? Thanks in advance.