[issue18212] No way to check whether Future is finished?

[Irit Katriel]

Change by Irit Katriel :


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resolution:  -> rejected
stage:  -> resolved
status: open -> closed

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[issue18212] No way to check whether Future is finished?

[Ram Rachum]

Ram Rachum added the comment:

What do you think about exposing this directly?

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[issue18212] No way to check whether Future is finished?

[Josh Rosenberg]

Josh Rosenberg added the comment:

Have an idea for a name? It already has a done method, which happens to cover 
the case of a Future having completed successfully, completed due to an 
exception or having been cancelled. Simply calling it finished(), 
completed() etc. isn't markedly different from done().

Exposing it as a property would be a bad idea largely because it would be 
inconsistent with the rest of the Future interface that is purely method based 
(I suspect largely because some of the behaviors must be method based to allow 
arguments related to timeout and they just kept up the pattern).

The main argument I can see *against* making this easier than Richard's 2-3 
part check (beyond the naming issue) is that it tends to encourage misuse of 
the library. Polling your Futures is a bad design 99% of the time (the same way 
loops over range(len(someseq)) are a bad idea most of the time; it's not that 
it doesn't work, it's that it's a backwards way to do it). In general:

1. If you need a single result now, you use future.result() directly
2. If you need to wait for a set of things to complete, you use:
  a. concurrent.futures.wait if you need all of them to finish before continuing
  b. concurrent.futures.as_completed if you can process them as they become 
available
3. If you don't have a loop processing worker results, but want the values to 
be used as they become available, you use future.add_done_callback() and let 
the result get processed asynchronously without your main thread becoming 
involved at all

Note: None of these cases really benefit from the existing status check 
methods. Instantaneous state checks are generally frowned upon in concurrent 
code, since program behavior can hinge on a microsecond race between the task 
and the checking thread, so polling is almost always worse than one of the more 
event driven approaches. I'm open to argument (I'm not omniscient), but I see 
this as making it even easier to do threading, rather than a legitimate 
improvement.

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nosy: +josh.rosenberg

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[issue18212] No way to check whether Future is finished?

[Josh Rosenberg]

Josh Rosenberg added the comment:

Ugh. Left out a key word:

I see this as making it even easier to do threading *badly*, rather than a 
legitimate improvement.

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[issue18212] No way to check whether Future is finished?

[Ram Rachum]

New submission from Ram Rachum:

I have a `Future` and I want to check whether it's in finished state. It seems 
like I don't have a method to do that, right? (I could emulate the existing 
methods for checking Future state, but that would mean fiddling with private 
variables.)

Why not just expose the Future state in a property that automatically acquires 
`self._condition`? (Instead of a horde of methods.)

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components: Library (Lib)
messages: 191126
nosy: cool-RR
priority: normal
severity: normal
status: open
title: No way to check whether Future is finished?
type: behavior
versions: Python 3.4

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[issue18212] No way to check whether Future is finished?

[Richard Oudkerk]

Richard Oudkerk added the comment:

Do you want something like

f.done() and not f.cancelled() and f.exception() is None

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nosy: +sbt

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[issue18212] No way to check whether Future is finished?

[Ram Rachum]

Ram Rachum added the comment:

I guess that'd be equivalent, yes.

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