Can I use map / imap / imap_unordered with functions with no arguments?

Question

Sometimes I need to use multiprocessing with functions with no arguments. I wish I could do something like:

from multiprocessing import Pool

def f():  # no argument
    return 1

# TypeError: f() takes no arguments (1 given)
print Pool(2).map(f, range(10))

I could do Process(target=f, args=()), but I prefer the syntax of map / imap / imap_unordered. Is there a way to do that?

Roark

Would you be willing to redefine f to take one argument and ignore it? – Royalroyalist

@inspectorG4dget: no, I'd rather avoid that – Roark

I have a feeling this is an XY problem. So, let's take a step back: what are you actually trying to do, for which you are trying to use this function? – Royalroyalist

The semantics of map imply that you're mapping a function to a sequence of inputs, so that's what you get. Whether you decide to ignore that argument, or create a wrapper function that's up to you. – Residuary

@inspectorG4dget: one example is to stress test a database or an API. I need to spawn N identical processes which will send requests repeatedly. – Roark

In that case, please edit your post to show a representative example of exactly what you're trying to do, so that we can give you an answer that is hopefully both meaningful as well as to your liking – Royalroyalist

@Royalroyalist So do you have another way of doing this that's not an XY problem? – Quetzal

Answer 1

map function's first argument should be a function and it should accept one argument. It is mandatory because, the iterable passed as the second argument will be iterated and the values will be passed to the function one by one in each iteration.

So, your best bet is to redefine f to accept one argument and ignore it, or write a wrapper function with one argument, ignore the argument and return the return value of f, like this

from multiprocessing import Pool

def f():  # no argument
    return 1

def throw_away_function(_):
    return f()

print(Pool(2).map(throw_away_function, range(10)))
# [1, 1, 1, 1, 1, 1, 1, 1, 1, 1]

You cannot use lamdba functions with pools because they are not picklable.

Darned

it does not have to be a function accepting one argument only. you can use partial if you want to pass more arguments. – Barbiturism

Answer 2

You can use pool.starmap() instead of .map() like so:

from multiprocessing import Pool

def f():  # no argument
    return 1

print Pool(2).starmap(f, [() for _ in range(10)])

starmap will pass all elements of the given iterables as arguments to f. The iterables should be empty in your case.

Mindoro

This can be rewritten as: print(Pool(2).starmap(f, [[]] * 10)). – Quotha

Answer 3

Is there anything wrong with using Pool.apply_async?

with multiprocessing.Pool() as pool:
    future_results = [pool.apply_async(f) for i in range(n)]
    results = [f.get() for f in future_results]

Emissive

This throws an error (Python 2.7): with multiprocessing.Pool() as pool: AttributeError: exit – Ramunni

Answer 4

For using the method pool, from library multiprocessing, you should provide an argument. Thus, you should use another method like Process from multiprocessing:

if __name__ == '__main__':
    p = Process(target=f)
    p.start()
    p.join()

English