Used to parallelize for-loops using parfor in Matlab? This package allows you to do the same in python. Take any normal serial but parallelizable for-loop and execute it in parallel using easy syntax. Don't worry about the technical details of using the multiprocessing module, race conditions, queues, parfor handles all that.
Tested on linux, Windows and OSX with python 3.10.
- Easy to use
- Using dill instead of pickle: a lot more objects can be used when parallelizing
- Progress bars are built-in
The work you want parfor to do is divided over a number of processes. These processes are started by parfor and put together in a pool. This pool is reused when you want parfor to do more work, or shut down when no new work arrives within 10 minutes.
A handle to each bit of work is put in a queue from which the workers take work. The objects needed to do the work are stored in a memory manager in serialized form (using dill) and the manager hands out an object to a worker when the worker is requesting it. The manager deletes objects automatically when they're not needed anymore.
When the work is done the result is sent back for collection in the main process.
pip install parfor
Parfor decorates a functions and returns the result of that function evaluated in parallel for each iteration of an iterator.
tqdm, dill
Objects passed to the pool need to be dillable (dill needs to serialize them). Generators and SwigPyObjects are examples
of objects that cannot be used. They can be used however, for the iterator argument when using parfor, but its
iterations need to be dillable. You might be able to make objects dillable anyhow using dill.register
or with
__reduce__
, __getstate__
, etc.
fun: function taking arguments: iteration from iterable, other arguments defined in args & kwargs
iterable: iterable or iterator from which an item is given to fun as a first argument
args: tuple with other unnamed arguments to fun
kwargs: dict with other named arguments to fun
total: give the length of the iterator in cases where len(iterator) results in an error
desc: string with description of the progress bar
bar: bool enable progress bar,
or a callback function taking the number of passed iterations as an argument
serial: execute in series instead of parallel if True, None (default): let pmap decide
length: deprecated alias for total
n_processes: number of processes to use,
the parallel pool will be restarted if the current pool does not have the right number of processes
**bar_kwargs: keyword arguments for tqdm.tqdm
list with results from applying the function 'fun' to each iteration of the iterable / iterator
<<
from time import sleep
a = 3
fun = []
for i in range(10):
sleep(1)
fun.append(a*i**2)
print(fun)
>> [0, 3, 12, 27, 48, 75, 108, 147, 192, 243]
<<
from time import sleep
from parfor import parfor
@parfor(range(10), (3,))
def fun(i, a):
sleep(1)
return a*i**2
print(fun)
>> [0, 3, 12, 27, 48, 75, 108, 147, 192, 243]
<<
@parfor(range(10), (3,), bar=False)
def fun(i, a):
sleep(1)
return a*i**2
print(fun)
>> [0, 3, 12, 27, 48, 75, 108, 147, 192, 243]
Parfor should never be executed during the import phase of a .py-file. To prevent that from happening
use the if __name__ == '__main__':
structure:
<<
from time import sleep
from parfor import parfor
if __name__ == '__main__':
@parfor(range(10), (3,))
def fun(i, a):
sleep(1)
return a*i**2
print(fun)
>> [0, 3, 12, 27, 48, 75, 108, 147, 192, 243]
or:
<<
from time import sleep
from parfor import parfor
def my_fun(*args, **kwargs):
@parfor(range(10), (3,))
def fun(i, a):
sleep(1)
return a*i**2
return fun
if __name__ == '__main__':
print(my_fun())
>> [0, 3, 12, 27, 48, 75, 108, 147, 192, 243]
pmap maps an iterator to a function like map does, but in parallel
<<
from parfor import pmap
from time import sleep
def fun(i, a):
sleep(1)
return a*i**2
print(pmap(fun, range(10), (3,)))
>> [0, 3, 12, 27, 48, 75, 108, 147, 192, 243]
If iterators like lists and tuples are too big for the memory, use generators instead. Since generators don't have a predefined length, give parfor the length (total) as an argument (optional).
<<
import numpy as np
c = (im for im in imagereader)
@parfor(c, total=len(imagereader))
def fun(im):
return np.mean(im)
>> [list with means of the images]
The function parfor decorates, it's used similarly to map
.
Split a long iterator in bite-sized chunks to parallelize
More low-level accessibility to parallel execution. Submit tasks and request the result at any time, (although necessarily submit first, then request a specific task), use different functions and function arguments for different tasks.