StreamWriter wraps (contains) a stream. It defines write and other respective methods to "encode" the data and pass the result to the stream. It exposes all other methods of the stream instance.

Pseudocode of the codecs.StreamWriter definition in Python2.5:

   1 class StreamWriter(Codec):
   2     def __init__(self, stream):
   3         ....
   4 
   5     def write(data):
   6         return stream.write(self.encode(data))

The StreamWriter.encode method is attached to the derived codec-specific class definitions at the time of the codec initialization. An excerpt from encodings.utf_8.StreamWriter:

   1 class StreamWriter(codecs.StreamWriter):
   2     encode = codecs.utf_8_encode