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The 'a' conversion type isn't documented in library/stdtypes.rst

I don’t think anyone could find anything wrong with the patch.

%r has note (5) saying "The precision determines the maximal number of characters used."
Does this apply to %a as well?
Other than that, LGTM

ISTM that it applies to both %s and %a, but the note is wrong/imprecise.
The precision determines where the string is truncated, but the actual number of characters is given by the field width.

Yeah, the note should probably be clarified to mention that if precision is specified, only the first precision characters will be shown. The field width is covered globally (for all conversion types) in note 4 above the table.
While we're at it, note 5 is:
 Precision (optional), given as a '.' (dot) followed by the precision. If specified as '*' (an asterisk), the actual width is read from the next element of the tuple in values, and the value to convert comes after the precision.
I think that the second sentence should have "the actual precision" instead "the actual width".

New changeset 9846f6463f23 by Eli Bendersky in branch '3.2':
Issue #12644: document the '%a' conversion in the old string formatting operations. Patch prepared together with Ezio Melotti
http://hg.python.org/cpython/rev/9846f6463f23
New changeset 80a3bf889cf6 by Eli Bendersky in branch 'default':
Merge from 3.2: Issue #12644: document the '%a' conversion in the old string formatting operations. Patch prepared together with Ezio Melotti
http://hg.python.org/cpython/rev/80a3bf889cf6 

Ezio, I've taken the liberty to adapt your patch with the suggested fixes and commit it to both 3.2 and 3.3
If everything is OK, this issue can be close (I'll do it in a few days if no one else does :-)

The commit looks fine, the issue can be closed.