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What is the purpose in touching fieldnames, either in tuple-izing it or in making it private and wrapped with a property. If someone wants to modify it, that is up to them. In any case, this change is not germane to the issue and could break code, so I would not make it.
wrong_fields could be calculated with
any(k for k in rowdict if k not in self._fieldset)
to stop on the first extra, if any.
That said, in 3.x, replacing
 wrong_fields = <long expression>
 if wrong_fields:
with
 if rowdict.keys() - self._fieldset:
should be even faster because the iteration, which will nearly always go to completion, is entirely in C (or whatever).
Does test/text_cvs have tests for DictWriter, both with and without rowdict errors? If so, or if added, I would be willing to commit a patch that simply added ._fieldset and used it as above for a set difference.
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