Python 3.5.0
PEP 0448 in addition to already implemented PEP 3132 make it very tempting to switch. If PEP 0448 had unpacking in comprehensions I'd switch. (Doesn't seem be a follow-up PEP just for that functionality yet.)https://www.python.org/dev/peps/pep-0448/
https://www.python.org/dev/peps/pep-3132/I'd be really nice to use this.  >>> [*range(i) for i in range(5)]

Instead of this monstrosity right now.  >>> [x for y in (range(i) for i in range(5)) for x in y]

Python 2.7 has some minor features that 3 dropped unfortunately, which still makes me hesitate.Such as filter keeping the type. In 3 it returns an iterator.  >>> filter(lambda x: x in 'ABC', 'ABCDEFA')
  'ABCA'

Or this mostly cosmetic feature.  >>> filter(lambda x: x[0] > x[1], ((1, 2), (4, 3)))
  >>> filter(lambda (x, y): x > y, ((1, 2), (4, 3))) # equivalent, error in 3

Also dropped. (It's slower than using the dedicated base64 module though.)  >>>'Python'.encode('base64')
  'UHl0aG9u\n'

Also, I like print.