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The csv expert listed in https://devguide.python.org/experts/ is marked as inactive, and I have never used the module. So you might need to ask for help on core-mentorship list.
The csv doc for Sniffer.sniff says "Analyze the given sample and return a Dialect subclass reflecting the parameters found." It is not clear to me whether 'the parameters found' is meant to be all possible parameters or just those found. So, to be conservative, I will initially treat this an a feature addition for the the next version, rather than a bug to also be fixed in current versions. It does seem like a reasonable request.