Python3 Stack implementation with List built-in
By far the biggest problem this code has is error handling. When popping or peeking from an empty stack this returns a string. this is really broken. For an example of why, suppose I add the string "Cannot peek into an empty stack." to the stack and then try to see what the top element is. I won't know if the answer I got was because the stack was empty, or because that string was in the stack. The solution to this is to raise an IndexError when the stack is empty. I also think that you shouldn't define either size or count, as the pythonic way of getting somethings length is to call len(x). You might also be interested to know that python lists have a pop function already, so you could just use that for your pop