Issue 26374: concurrent_futures Executor.map semantics better specified in docs (original) (raw)
Created on 2016-02-17 12:26 by F.D. Sacerdoti, last changed 2022-04-11 14:58 by admin. This issue is now closed.
Messages (7)
Author: F.D. Sacerdoti (F.D. Sacerdoti)
Date: 2016-02-17 12:26
Hello,
My colleague and I have both written parallel executors for the concurrent_futures module, and are having an argument, as described in the dialog below. To resolve, I would like to add "order of results is undefined" to disambiguate the docs for "map(func, *iterables, timeout=None)".
DISCUSSION
Q: Correct Semantics to return results out of order? JH: No, breaks API as stated Rebut: order is undefined, concurrent_futures specifies map() returns an iterator, where builtin map returns a list. Q: Does it break the spirit of the module? A: No, I believe one of the best things about doing things async is the dataflow model: do the next thing as soon as its inputs are ready. Q: Should we hold up the caller in all cases when there are stragglers, i.e. elements that compute slower? A: No, the interface should allow both modes.
def james_map(exe, fn, *args): return iter( sorted( list( exe.map( fn, *args ) ) ) )
Author: Mark Dickinson (mark.dickinson) * 
Date: 2016-02-18 18:57
The documentation says: "Equivalent to map(func, *iterables)". I believe that that equivalency implies that the ordering is defined, so it would be incorrect to add "order of results is undefined" to the documentation.
Author: Mark Dickinson (mark.dickinson) *
Date: 2016-02-18 19:00
Note also this code snippet from PEP 3148:
for number, prime in zip(PRIMES, executor.map(is_prime,
PRIMES)):The use of zip here suggests strongly that the intention is that the order of the map result is well-defined.
It's possible that the docs should be updated to make the ordering requirement clearer.
Author: Mark Dickinson (mark.dickinson) *
Date: 2016-02-19 07:59
I just noticed this point, which may be confusing things:
Rebut: order is undefined, concurrent_futures specifies map() returns an iterator, where builtin map returns a list.
In Python 3, the built-in map function returns an iterator, not a list.
Author: Karthikeyan Singaravelan (xtreak) *
Date: 2018-09-23 16:07
There were some improvements made that clarify differences between builtin map with https://bugs.python.org/issue32306 and https://github.com/python/cpython/commit/a7a751dd7b08a5bb6cb399c1b2a6ca7b24aba51d
Thanks
Author: Brian Quinlan (bquinlan) *
Date: 2019-05-06 19:32
Can we close this bug then?
Author: Karthikeyan Singaravelan (xtreak) *
Date: 2019-05-06 19:42
I would propose closing since the original doc issue regarding order and map in Python 3 is resolved. Just to add there is a PR to make map less eager : https://github.com/python/cpython/pull/707/
History
Date
User
Action
Args
2022-04-11 14:58:27
admin
set
github: 70562
2019-05-06 19:43:33
bquinlan
set
status: open -> closed
stage: resolved
2019-05-06 19:42:01
xtreak
set
messages: +
2019-05-06 19:32:00
bquinlan
set
messages: +
2018-09-23 16:07:47
xtreak
set
nosy: + xtreak
messages: +
2016-02-19 07:59:55
mark.dickinson
set
messages: +
2016-02-19 07:52:09
mark.dickinson
set
assignee: docs@python
components: + Documentation, - Library (Lib)
nosy: + docs@python
2016-02-18 19:00:01
mark.dickinson
set
messages: +
2016-02-18 18:57:06
mark.dickinson
set
nosy: + mark.dickinson
messages: +
2016-02-18 18:50:44
SilentGhost
set
nosy: + bquinlan
components: + Library (Lib)
2016-02-17 12:26:30
F.D. Sacerdoti
create