input: Write a python function to implement a basic stack class with methods to check if it is empty, push an item, pop an item, peek at the top item, and get the size of the stack.
  assert __name__ == "__main__":
      stack = Stack()
      assert stack.isEmpty() == True
      stack.push(1)
      stack.push(2)
      assert stack.peek() == 2
      assert stack.size() == 2
      assert stack.pop() == 2
      assert stack.pop() == 1
      assert stack.isEmpty() == True
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):
          return self.items.pop() if not self.isEmpty() else None
  
      def peek(self):
          return self.items[-1] if not self.isEmpty() else None
  
      def size(self):
          return len(self.items)
  
  if __name__ == "__main__":
      stack = Stack()
      assert stack.isEmpty() == True
      stack.push(1)
      stack.push(2)
      assert stack.peek() == 2
      assert stack.size() == 2
      assert stack.pop() == 2
      assert stack.pop() == 1
      assert stack.isEmpty() == True