Pool.apply_async(): nested function is not executed

Question

I am getting familiar with Python's multiprocessing module. The following code works as expected:

#outputs 0 1 2 3
from multiprocessing import Pool
def run_one(x):
    print x
    return

pool = Pool(processes=12)
for i in range(4):
    pool.apply_async(run_one, (i,))
pool.close()
pool.join()

Now, however, if I wrap a function around the above code, the print statements are not executed (or the output is redirected at least):

#outputs nothing
def run():
    def run_one(x):
        print x
        return    

    pool = Pool(processes=12)
    for i in range(4):    
        pool.apply_async(run_one, (i,))
    pool.close()
    pool.join()

If I move the run_one definition outside of run, the output is the expected one again, when I'm calling run():

#outputs 0 1 2 3
def run_one(x):
    print x
    return

def run():    
    pool = Pool(processes=12)
    for i in range(4):       
        pool.apply_async(run_one, (i,))
    pool.close()
    pool.join()

What am I missing here? Why isn't the second snippet printing anything? If I simply call the run_one(i) function instead of using apply_async, all the three codes output the same.

Darkonaut · Accepted Answer · 2019-10-27T12:14:44.333

Pool needs to pickle (serialize) everything it sends to its worker-processes. Pickling actually only saves the name of a function and unpickling requires re-importing the function by name. For that to work, the function needs to be defined at the top-level, nested functions won't be importable by the child and already trying to pickle them raises an exception:

from multiprocessing.connection import _ForkingPickler

def run():
    def foo(x):
        pass
    _ForkingPickler.dumps(foo)  # multiprocessing custom pickler;
                                # same effect with pickle.dumps(foo)

run()
# Out:
Traceback (most recent call last):
...
AttributeError: Can't pickle local object 'run.<locals>.foo'

The reason why you don't see an exception is, because Pool already starts catching exceptions during pickling tasks in the parent and only re-raises them when you call .get() on the AsyncResult object you immediately get when you call pool.apply_async().

That's why (with Python 2) you better always use it like this, even if your target-function doesn't return anything (still returns implicit None):

    results = [pool.apply_async(foo, (i,)) for i in range(4)]
    # `pool.apply_async()` immediately returns AsyncResult (ApplyResult) object
    for res in results:
        res.get()

Non-async Pool-methods like Pool.map() and Pool.starmap() use the same (asynchronous) low-level functions under the hood like their asynchronous siblings, but they additionally call .get() for you, so you will always see an exception with these methods.

Python 3 has an error_callback-parameter for asynchronous Pool-methods you can use instead to handle exceptions.

Pool.apply_async(): nested function is not executed

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