Stack in Python

class Node:
   def __init__(self, value, next):
       self.value = value
       self.next = next

class Stack:
   def __init__(self):
       self.head = None
       self.size = 0
   
   def isEmpty(self):
       return self.size <= 0

   def peek(self):
       if self.isEmpty():
           raise IndexError
       return self.head.value
   
   def push(self, value):
       if self.isEmpty():
           self.head = Node(value, None)
           self.size += 1
           return
       tmp = Node(value, self.head)
       self.head = tmp
       self.size += 1
   
   def pop(self):
       if self.isEmpty():
           raise IndexError
       tmp = self.head
       self.head = self.head.next
       self.size -= 1
       return tmp.value
   
   def poll(self):
       if self.isEmpty():
           return None
       tmp = self.head
       self.head = self.head.next
       self.size -= 1
       return tmp.value
   
   def __str__(self):
       res = "["
       tmp = self.head
       while tmp is not None:
           res += str(tmp.value)
           if tmp.next is not None:
               res += ", "
           tmp = tmp.next
       res += "]"
       return res