Stack class

Using a list for data storage. The list only expands and never contracts

peek(where) is a little more general than the normal peek() function:
peek() = peek(0) shows the item at the top of the stack
peek(1) shows the item just below the top of the stack
peek(-1) shows the bottom of the stack
peek(-2) shows the the item one above the bottom of the stack

class MyStack:
    def __init__(self,store_immediate_list = None):
        self.storage = []
        self.filled = 0
        # if there's anything to store immediately, push all items in that list
        if store_immediate_list:
            for an_item in store_immediate_list:
                self.push(an_item)
        
    def size(self):
        return self.filled
    
    def push(self,item):
        if self.filled == len(self.storage):
            self.storage.append(item)
        else:
            self.storage[self.filled] = item
        self.filled += 1
        
    def pop(self):
        if self.filled == 0:
            return None
        self.filled -= 1
        return self.storage[self.filled]
    
    def peek(self,where = 0):
        if where >= 0:
            if where >= self.filled:
                return None
            return self.storage[self.filled - 1 - where]
        else:
            where = -where
            if where > self.filled:
                return None
            return self.storage[where-1]

# Testing
a = MyStack(list(range(1,4)))
print ('size = %d, should be 3' % a.size())
print ('peek() = %d, should be 3' % a.peek())
print ('peek(2)= %d, should be 1' % a.peek(2))
print ('peek(-2) = %d, should be 2' % a.peek(-2))
print ('pop() = %d, should be 3' % a.pop())
a.push(12)
print ('push(12), pop() = %d, should be 12' % a.pop())
a.pop()
print ('pop(), pop() = %d, should be 1' % a.pop())
print ('pop() should be None, is ', a.pop())

size = 3, should be 3
peek() = 3, should be 3
peek(2)= 1, should be 1
peek(-2) = 2, should be 2
pop() = 3, should be 3
push(12), pop() = 12, should be 12
pop(), pop() = 1, should be 1
pop() should be None, is  None