code: def for_format(filename, format=None, writable=False, prefix_dir=None):
      
      ext = op.splitext(filename)[1].lower()
      try:
          _format = supported_formats[ext]
      except KeyError:
          _format = None
  
      format = get_default(format, _format)
  
      if format is None:
          io = MeshIO.any_from_filename(filename, prefix_dir=prefix_dir)
  
      else:
          if not isinstance(format, basestr):
              raise ValueError('ambigous suffix! (%s -> %s)' % (ext, format))
  
          if format not in io_table:
              raise ValueError('unknown output mesh format! (%s)' % format)
  
          if writable and ('w' not in supported_capabilities[format]):
              output_writable_meshes()
              msg = 'write support not implemented for output mesh format "%s",' \
                    ' see above!' \
                    % format
              raise ValueError(msg)
  
          if prefix_dir is not None:
              filename = op.normpath(op.join(prefix_dir, filename))
  
          io = io_table[format](filename)
  
      return io
docs: Create a MeshIO instance for file `filename` with forced `format`.
  
      Parameters
      ----------
      filename : str
          The name of the mesh file.
      format : str
          One of supported formats. If None,
          :func:`MeshIO.any_from_filename()` is called instead.
      writable : bool
          If True, verify that the mesh format is writable.
      prefix_dir : str
          The directory name to prepend to `filename`.
  
      Returns
      -------
      io : MeshIO subclass instance
          The MeshIO subclass instance corresponding to the `format`.