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The following code:
>>> "{0:s}".format([1,2,3])
no longer works in Python 3.4b1, and gives the following exception:
>>> "{0:s}".format([1,2,3])
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: non-empty format string passed to object.__format__
This worked previously in Python 2.6-3.3:
>>> "{0:s}".format([1,2,3])
'[1, 2, 3]'
If this is a deliberate change, it should be included in the 'What's new in Python 3.4'

See issue 7994 and issue 9856. This behavior has been deprecated for a while, and is now an error.

Oops, I missed the "should be documented" part of your message. That's true, and I'll leave this open. But I'll change it to a doc request and assign it accordingly.

I believe this has been added to the 3.4 What's New already:
object.__format__() no longer accepts non-empty format strings, it now raises a TypeError instead. Using a non-empty string has been deprecated since Python 3.2. This change has been made to prevent a situation where previously working (but incorrect) code would start failing if an object gained a __format__ method, which means that your code may now raise a TypeError if you are using an 's' format code with objects that do not have a __format__ method that handles it. See bpo-7994 for background
Closing as resolved.