Basically any time I need to unpack inside a lambda (e.g. functional programming) and have to use something like itertools.groupby(), I run into trouble with Python 3. For example:

  from itertools import groupby
  def groupby_unsorted(items, key):
    return map(lambda (g, l): (g, map(lambda (k, v): v, l)), groupby(sorted(map(lambda v: (key(v), v), items)), lambda (k, v): k))
  print(groupby_unsorted([1, 2, 3, 4], lambda v: v % 2))