[issue41577] Cannot use ProcessPoolExecutor if in a decorator?

[Kyle Stanley]

Kyle Stanley <aeros167 at gmail.com> added the comment:

Due to the way pickle works, it's not presently possible to serialize wrapped functions directly, at least not in a way that allows you to pass it to executor.submit() within the inner function (AFAICT). I'm also not certain what it would involve to provide that, or if it would be feasible to do so in a manner that would be backwards compatible.

In the meantime, this is a decent work-around:

```
from concurrent import futures
import random

class PPEWrapper:
    def __init__(self, func, num_proc=4):
        self.fn = func
        self.num_proc = num_proc
    
    def __call__(self, *args, **kwargs):
        with futures.ProcessPoolExecutor() as executor:
            fs = []
            for i in range(self.num_proc):
                fut = executor.submit(self.fn, *args, **kwargs)
                fs.append(fut)
            result = []
            for f in futures.as_completed(fs):
                result.append(f.result())
        return result


def _get_random_int():
    return random.randint(0, 100)

# it's often quite useful anyways to have a parallel and non-parallel version
# (for testing and devices that don't support MP)
get_random_int = PPEWrapper(_get_random_int, num_proc=4)

if __name__ == "__main__":
    result = get_random_int()
    print(result)
```

This doesn't allow you to use the decorator syntax, but it largely provides the same functionality. That being said, my familiarity with the pickle protocol isn't overly strong, so the above is mostly based on my own recent investigation. There could very well be a way to accomplish what you're looking for in a way that I was unable to determine.

----------
nosy: +aeros, alexandre.vassalotti, pitrou

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<https://bugs.python.org/issue41577>
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