code: def register_compressor(ext, callback):
      
      if not (ext and ext[0] == '.'):
          raise ValueError('ext must be a string starting with ., not %r' % ext)
      if ext in _COMPRESSOR_REGISTRY:
          logger.warning('overriding existing compression handler for %r', ext)
      _COMPRESSOR_REGISTRY[ext] = callback
docs: Register a callback for transparently decompressing files with a specific extension.
  
      Parameters
      ----------
      ext: str
          The extension.
      callback: callable
          The callback.  It must accept two position arguments, file_obj and mode.
  
      Examples
      --------
  
      Instruct smart_open to use the identity function whenever opening a file
      with a .xz extension (see README.rst for the complete example showing I/O):
  
      >>> def _handle_xz(file_obj, mode):
      ...     import lzma
      ...     return lzma.LZMAFile(filename=file_obj, mode=mode, format=lzma.FORMAT_XZ)
      >>>
      >>> register_compressor('.xz', _handle_xz)