In Python 3, trying to json-dump a dict with keys of different types fails with a TypeError when sort_keys is specified:

python2.7
===========

Python 2.7.10 (default, May 29 2015, 10:02:30) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> json.dumps({1 : 42, "foo" : "bar", None : "nada"}, sort_keys = True)
'{"null": "nada", "1": 42, "foo": "bar"}'

python3.5
============

Python 3.5.0 (default, Oct  5 2015, 12:03:13) 
[GCC 4.8.5] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> json.dumps({1 : 42, "foo" : "bar", None : "nada"}, sort_keys = True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.5/json/__init__.py", line 237, in dumps
    **kw).encode(obj)
  File "/usr/lib64/python3.5/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib64/python3.5/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
TypeError: unorderable types: str() < int()

Note that the documentation explicitly allows keys of different, if basic, types:

  If skipkeys is True (default: False), then dict keys that are not of a basic type (str, int, float, bool, None) will be skipped instead of raising a TypeError.

As all they keys are dumped as strings, a simple solution would be to sort after converting to strings. Looking closely at the output of Python 2, the sort order is a bit strange!