What calls on_headers in this example? Coming from twisted, that seems like
dataReceived's responsibility, but given your introductory paragraph that's
not actually what goes on here?
On Sat, Oct 13, 2012 at 7:07 PM, Ben Darnell
On Fri, Oct 12, 2012 at 11:14 PM, Antoine Pitrou
wrote: On Fri, 12 Oct 2012 15:11:54 -0700 Guido van Rossum
wrote: 2. Method dispatch callbacks:
Similar to the above, the reactor or somebody has a handle on your object, and calls methods that you've defined when events happen e.g. IProtocol's dataReceived method
While I'm sure it's expedient and captures certain common patterns well, I like this the least of all -- calling fixed methods on an object sounds like a step back; it smells of the old Java way (before it had some equivalent of anonymous functions), and of asyncore, which (nearly) everybody agrees is kind of bad due to its insistence that you subclass its classes. (Notice how subclassing as the prevalent approach to structuring your code has gotten into a lot of discredit since 1996.)
But how would you write a dataReceived equivalent then? Would you have a "task" looping on a read() call, e.g.
@task def my_protocol_main_loop(conn): while
: try: data = yield conn.read(1024) except ConnectionError: conn.close() break I'm not sure I understand the problem with subclassing. It works fine in Twisted. Even in Python 3 we don't shy away from subclassing, for example the IO stack is based on subclassing RawIOBase, BufferedIOBase, etc.
Subclassing per se isn't a problem, but requiring a single dataReceived method per class can be awkward. Many protocols are effectively state machines, and modeling each state as a function can be cleaner than a big if/switch block in dataReceived. For example, here's a simplistic HTTP client using tornado's IOStream:
from tornado import ioloop from tornado import iostream import socket
def send_request(): stream.write("GET / HTTP/1.0\r\nHost: friendfeed.com\r\n\r\n") stream.read_until("\r\n\r\n", on_headers)
def on_headers(data): headers = {} for line in data.split("\r\n"): parts = line.split(":") if len(parts) == 2: headers[parts[0].strip()] = parts[1].strip() stream.read_bytes(int(headers["Content-Length"]), on_body)
def on_body(data): print data stream.close() ioloop.IOLoop.instance().stop()
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0) stream = iostream.IOStream(s) stream.connect(("friendfeed.com", 80), send_request) ioloop.IOLoop.instance().start()
Classes allow and encourage broader interfaces, which are sometimes a good thing, but interact poorly with coroutines. Both twisted and tornado use separate callbacks for incoming data and for the connection being closed, but for coroutines it's probably better to just treat a closed connection as an error on the read. Futures (and yield from) give us a nice way to do that.
-Ben _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas
-- cheers lvh