code: def _boundary_value(self) -> str:
          
          # Refer to RFCs 7231, 7230, 5234.
          #
          # parameter      = token "=" ( token / quoted-string )
          # token          = 1*tchar
          # quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
          # qdtext         = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
          # obs-text       = %x80-FF
          # quoted-pair    = "\" ( HTAB / SP / VCHAR / obs-text )
          # tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
          #                  / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
          #                  / DIGIT / ALPHA
          #                  ; any VCHAR, except delimiters
          # VCHAR           = %x21-7E
          value = self._boundary
          if re.match(self._valid_tchar_regex, value):
              return value.decode('ascii')  # cannot fail
  
          if re.search(self._invalid_qdtext_char_regex, value):
              raise ValueError("boundary value contains invalid characters")
  
          # escape %x5C and %x22
          quoted_value_content = value.replace(b'\\', b'\\\\')
          quoted_value_content = quoted_value_content.replace(b'"', b'\\"')
  
          return '"' + quoted_value_content.decode('ascii') + '"'
docs: Wrap boundary parameter value in quotes, if necessary.
  
          Reads self.boundary and returns a unicode sting.