On 7 Nov 2009, at 12:40, Antoine Pitrou wrote:
To Antoine's Twisted comment, I don't see a direct comparison. From
my
understanding Twisted's Deferred objects are ways to have callbacks
executed once an async event occurs, not to help execute code
concurrently.
Well, waiting for concurrently executing code to terminate *is* a case
of waiting for an async event to happen.
Deferred objects are a generic mechanism to chain reactions to
termination or failure of code. Whether the event your Deferred reacts
to is "async" or not is really a matter of how you use it (and of how
you define "async" -- perhaps you meant "I/O" but Deferreds are not
specialized for I/O).
They do seem specialized for continuation-passing style programming
though. As far as I can tell from the docs (http://python.net/crew/mwh/apidocs/twisted.internet.defer.Deferred.html
), the only way to process the results of a Deferred is my installing
a callback.
Maybe you could outline (at a super-high-level) how you would
implement my URL-downloading example using a Deferred-based API? Maybe
something like:
def print_success(result, url):
print('%r page is %d bytes' % (url, len(result)))
def print_failure(exception, url):
print('%r generated an exception: %s' % (url, exception))
with ThreadedDeferredMaker(max_threads=5) as dm
deferreds = []
for url in URLS:
deferred = dm.defer(load_url, url)
deferred. addCallbacks(print_success, print_failure, url=url)
deferred.unpause()
deferreds.append(deferred)
dm.wait_for_all_to_complete(deferreds)
The semantics aren't quite the same because the order of the output
would be non-deterministic in this case. OTOH, you are going to get
intermediate results as they become available, which is cool.
Cheers,
Brian
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