Sorting mixed type lists in Python 3

Because this bit me harder than I was ready for, I thought I'd make a note of it for the next victim.

In Python 2, suppose you have this:

Python 2.7.5
>>> items = [(1, 'A number'), ('a', 'A letter'), (2, 'Another number')]

Sorting them, without specifying how, will automatically notice that it contains tuples:

Python 2.7.5
>>> sorted(items)
[(1, 'A number'), (2, 'Another number'), ('a', 'A letter')]

This doesn't work in Python 3 because comparing integers and strings is not allowed. E.g.:

Python 3.3.3
>>> 1 < '1'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: int() < str()

You have to convert them to stings first.

Python 3.3.3
>>> sorted(items, key=lambda x: str(x[0]))
[(1, 'A number'), (2, 'Another number'), ('a', 'A letter')]

If you really need to sort by 1 < '1' this won't work. Then you need a more complex key function. E.g.:

Python 3.3.3
>>> def keyfunction(x):
...   v = x[0]
...   if isinstance(v, int): v = '0%d' % v
...   return v
...
>>> sorted(items, key=keyfunction)
[(1, 'A number'), (2, 'Another number'), ('1', 'Actually a string')]

That's really messy but the best I can come up with at past 4PM on Friday.