Davin Potts added the comment:

This appears to be working as designed.  The purpose of the callback is to 
receive a single argument (nominally, the ready result from the map operation) 
and perform its task (whatever it may be) very quickly so as not to further 
delay the handing back of results from the map operation.

Attempting to read into your comments, it sounds like you were perhaps 
expecting the return from the callback to appear somewhere?  If that was your 
expectation then no, that is not the purpose of a callback here.  Any 
transformation of input data to the final output data is performed by the 
function being applied by the map.  While there are many potential uses for a 
callback, its stated requirement is that it be as quick an operation as 
possible as opposed to something that further transforms the results or 
anything like a "reduce" operation which is not necessarily quick.

If you think this needs clarification in the documentation, please suggest 
something?

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue29108>
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