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 ) ) ) )

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.

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.

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.

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

Can we close this bug then?

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/