Really difficult to track are issues caused by things like the map function now returning an iterator object. I.e. the content of the iterator is empty on second use.

  Python 3.4.3 (default, Oct 14 2015, 20:28:29) 
  [GCC 4.8.4] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> x = map(lambda v: v + 1, [1, 2, 3])
  >>> list(x)
  [2, 3, 4]
  >>> list(x)
  []