Simple Python stack

Stack.py

class Stack:
    def __init__(self):
        self.items=[]

    def isEmpty(self):
        return self.items==[]

    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[len(self.items) - 1] if not self.isEmpty() else None

    def size(self):
        return len(self.items)