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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