yield from - redundant syntax?

Greg Ewing is putting together a proposal to adding a "yield from" keyword to Python.

The Spam Server code he provides as a supporting example can already be achieved using the Fibra scheduler, without the new syntax. Notice that Fibra allows exceptions to propagate from an inner task to the outer task.

Fibra also allows an inner task to return values to its parent task, by yielding a "Return(value)" instance. This is what the line_recv task uses to continually yield lines from a socket to the parent task, while maintaining its own internal state.

Perhaps the "yield from" syntax is redundant. Fibra doesn't need it, and Fibra fulfilles the main use-case behind the PEP.

The code below is a direct translation of the Spam Server example, using Fibra.

import socket
import fibra
import fibra.net

port = 4200

def handler(transport):
    try:
        while True:
            try:
                line = yield transport.recv_line()
                n = parse_request(line)
                yield transport.send_line("100 SPAM FOLLOWS")
                for i in xrange(n):
                    yield transport.send_line("spam glorious spam")
            except BadRequest:
                yield transport.send_line("400 WE ONLY SERVE SPAM")
    except socket.error:
        pass 
    
class BadRequest(Exception):
    pass
  
def parse_request(line):
    tokens = line.split()
    if len(tokens) != 2 or tokens[0] != "SPAM":
      raise BadRequest
    try:
      n = int(tokens[1])
    except ValueError:
      raise BadRequest
    if n < 1:
      raise BadRequest
    return n

schedule = fibra.schedule()
schedule.install(fibra.net.listen(('localhost', port), handler))
schedule.run()

Update: modified handler to use a new transport class.