两个长字符串的Python'is'运算符

Question

a = 'This is a very long string. This is a very long string. No?'
b = 'This is a very long string. This is a very long string. No?'

print(id(a), id(b))
print(a is b)

在我的电脑上，结果是：

4467772080 4467772080
True

据我所知，Python 只缓存短字符串。但是为什么这么长的字符串，它仍然只保留一个副本？其实我把它改成了一个很长的字符串（甚至比1024还长），a和b仍然指向同一个字符串对象。请纠正我哪里错了？