initializing basic stack, but errors i do not comprehend - oop

Question

i am trying to initialize a basic stack - with push, and pop function.
the problems comes in while testing. you will notice in the code that i have pushed 2 times, so a print of the stack should display [5,5] whereas it is displaying None. I could tinker with the code to make it work eventually, but then i will not fully understand the underlying concepts and the error of my ways. so i ask for advice and pointers.
please check out these codes and tell me what i am doing wrong.
this is the code with the class with all it's functions, it is named stack_class:
class Stack:
def __init__(self):
self._values = []
print ('Stack initialized...')
return
def push(self, var):
ok = self._values.append(var)
return ok
def pop(self):
self.stack.pop()
def __str__(self):
output = "{0}".format(self.ok)
return output
this is the testing code:
from stack_class import Stack
ob_1 = Stack()
ob_1.push(5)
print(ob_1.push(5))

Answer 1

What happens is that append method doesn't return anything.
print [].append(1)
>>> None
It does its job, but doesn't return anything, that's why you are getting None in variable ok. I think you want to return the _values instead:
def push(self, var):
self._values.append(var)
return self._values
Output:
Stack initialized...
[5, 5]
Also, it's the first time I read about empty return convention. It's not necessary.