>>> import datetime, pprint
>>> pprint.pformat({datetime.datetime.now(): 1, None: 1})

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/pprint.py", line 63, in pformat
    return PrettyPrinter(indent=indent, width=width, depth=depth).pformat(object)
  File "/usr/lib/python2.7/pprint.py", line 122, in pformat
    self._format(object, sio, 0, 0, {}, 0)
  File "/usr/lib/python2.7/pprint.py", line 140, in _format
    rep = self._repr(object, context, level - 1)
  File "/usr/lib/python2.7/pprint.py", line 226, in _repr
    self._depth, level)
  File "/usr/lib/python2.7/pprint.py", line 238, in format
    return _safe_repr(object, context, maxlevels, level)
  File "/usr/lib/python2.7/pprint.py", line 280, in _safe_repr
    for k, v in _sorted(object.items()):
  File "/usr/lib/python2.7/pprint.py", line 82, in _sorted
    return sorted(iterable)
TypeError: can't compare datetime.datetime to NoneType

This is issue is kind of related to http://bugs.python.org/issue14998
but that talks only python 3.2+

This also applies to sets.

Actually, it applies to datetime, which is is almost unique among python2 datatypes in not being comparable to other types by default.  I'm not sure this is worth fixing in 2.7.

Complex numbers also aren't sortable.

> I'm not sure this is worth fixing in 2.7.

I agree.  That ship sailed a long time ago.