Suppose you have an exception object acquired from somewhere.  The exception is no longer active on the stack.  Now you want to format the exception like format_exception would (to log it, perhaps).  To do this you apparently need to call:

  format_exception(type(exc), exc, exc.__traceback__)

This seems redundant, so it would be nice to have a simpler call.  Too bad the name format_exception is already taken.  But, leaving that aside, the above is not documented in the traceback module.  The last example shows the (type(exc), exc) call form for format_exception_only, but that's as close as it gets.

Why not reuse format_exception(exc)?
in 2.7 you could write "raise type, value, traceback", or "raise value".
I suggest that traceback.format_exception take the same route, and accept both (type, value, traceback) and (value).

Sounds OK to me, though I am worried others will think that kind of variable signature (where the type of the first argument depends on the number of arguments passed) is bad.