code: def _is_extra_source(asset, field, map):
          
          # If we have an extra source with a column called "price", only look
          # at it if it's on something like palladium and not AAPL (since our
          # own price data always wins when dealing with assets).
  
          return not (field in BASE_FIELDS and
                      (isinstance(asset, (Asset, ContinuousFuture))))
docs: Internal method that determines if this asset/field combination
          represents a fetcher value or a regular OHLCVP lookup.