# File lib/aws/s3/data_options.rb, line 146
        def initialize write_block
          unless write_block.arity == 2
            msg = "a write block must accept 2 yield params: a buffer and "
            msg << "a number of bytes to write"
            raise ArgumentError, msg
          end
          @write_block = write_block
          @eof = false
        end