input: Write a function to implement a stack using class in Python.
  assert s.isEmpty() == True
  s.push(4)
  s.push('dog')
  assert s.peek() == 'dog'
  s.push(True)
  assert s.size() == 3
  assert s.isEmpty() == False
  s.push(8.4)
  assert s.pop() == 8.4
  assert s.pop() == True
  assert s.size() == 2
output: class Stack:
      def __init__(self):
          self.items = []
  
      def isEmpty(self):
          return len(self.items) == 0
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          if not self.isEmpty():
              return self.items.pop()
          else:
              return None
  
      def peek(self):
          if not self.isEmpty():
              return self.items[-1]
          else:
              return None
  
      def size(self):
          return len(self.items)
  
  # Test cases
  s = Stack()
  assert s.isEmpty() == True
  s.push(4)
  s.push('dog')
  assert s.peek() == 'dog'
  s.push(True)
  assert s.size() == 3
  assert s.isEmpty() == False
  s.push(8.4)
  assert s.pop() == 8.4
  assert s.pop() == True
  assert s.size() == 2