docs: def _get_format(format, fname, inp=None):
      """Try to guess markup format of given input.
  
      Args:
          format: explicit format override to use
          fname: name of file, if a file was used to read `inp`
          inp: optional bytestring to guess format of (can be None, if markup
              format is to be guessed only from `format` and `fname`)
      Returns:
          guessed format (a key of fmt_to_exts dict)
      Raises:
          AnyMarkupError if explicit format override has unsupported value
              or if it's impossible to guess the format
      """
      fmt = None
      err = True
  
      if format is not None:
          if format in fmt_to_exts:
              fmt = format
              err = False
      elif fname:
          # get file extension without leading dot
          file_ext = os.path.splitext(fname)[1][len(os.path.extsep):]
          for fmt_name, exts in fmt_to_exts.items():
              if file_ext in exts:
                  fmt = fmt_name
                  err = False
  
      if fmt is None:
          if inp is not None:
              fmt = _guess_fmt_from_bytes(inp)
              err = False
  
      if err:
          err_string = 'Failed to guess markup format based on: '
          what = []
          for k, v in {format: 'specified format argument',
                       fname: 'filename', inp: 'input string'}.items():
              if k:
                  what.append(v)
          if not what:
              what.append('nothing to guess format from!')
          err_string += ', '.join(what)
          raise AnyMarkupError(err_string)
  
      return fmt