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The text speaks about the regular case of a second character that
is not a valid alignment character, e.g.:
 format(3.222, ".2f")
Clearly the '2' fulfills this criterion, so the parser knows that the
leading '.' is *not* a fill character. This is all that the text says.
But even in your irregular case the text is still correct: After
it has been established that [[fill]align] is not present you have
to match the *whole string* with the rest of the grammar:
 [sign][#][0][width][,][.precision][type]
There is no match for "xx10d", hence the error.
BTW, I think this is out of scope for the tracker now. If you
have further questions, you could ask on:
 http://mail.python.org/mailman/listinfo/python-list