It seems to be a case of leaky abstraction, where the way python caches small objects becomes visible, another example:

  >>> a = 'hn'
  >>> a is 'hn'
  True
  >>> a = ''.join(['h', 'n'])
  >>> a is 'hn'
  False
  >>> a
  'hn'
  >>> a = 'h' + 'n'
  >>> a is 'hn'
  True

Edit: found another interesting case

  >>> a = 1
  >>> b = 1
  >>> a is b
  True
  >>> a = 500
  >>> b = 500
  >>> a is b
  False