Python: object variable vs standalone variable

Question

I'm writing a stack class MyStack, implemented as a list, where each element is a dictionary with two fields ('a' and 'b'). Each of these dictionary fields is itself a list (initialized as empty lists in push()). Apart from push() and pop() (the latter skipped for readability here) I intend to add elements to these lists (the dictionary fields) for the element at the top of the stack (index [-1]).

The problem is the value gets added to both dictionary fields, not just the selected one. However, I then test this with a standalone variable,standalone_stack ,that I test is identical to the stack variable of object MyStack. Surprisingly, I get what I want - I append only to the selected field. Here's the code:

 class MyStack(object):
    def __init__(self):
        self.stack = []

    def push(self):
        self.stack.append(dict.fromkeys(['a', 'b'], []))

    def add(self, field, value):
        self.stack[-1][field].append(value)

    def __str__(self):
        return str(self.stack)

def lists_the_same(list1, list2):
    if list1 == list2:
        print 'Yes, the same'
    else:
        print 'No, not the same'

# Create essentially the same data in two different ways:
ms = MyStack()
standalone_stack = [{'a': [], 'b': []}]
ms.push()
print ms
print standalone_stack
lists_the_same(ms.stack, standalone_stack)

# Now modify both:
field = 'a'
value = 2
ms.add(field=field, value=value)
standalone_stack[-1][field].append(value)
print ms
print standalone_stack
lists_the_same(ms.stack, standalone_stack)

My output with Python 2.7.6 is below:

[{'a': [], 'b': []}]
[{'a': [], 'b': []}]
Yes, the same
[{'a': [2], 'b': [2]}]
[{'a': [2], 'b': []}]
No, not the same

What I need help with is:

• Why there's this odd difference in behavior when the same list is a field of an object (self.stack) or a standalone variable standalone_stack? Even pointing out Python related keywords would help, since I'm not able to google this out for some time already.
• If I could make my question more clear. This is my first post here.
• Maybe I'm thinking about this backwards and you'd implement this differently, possibly in a more Pythonic way.