这可能是一个微不足道的问题,但我如何在python中并行化下面的循环?

# setup output lists
output1 = list()
output2 = list()
output3 = list()

for j in range(0, 10):
    # calc individual parameter value
    parameter = j * offset
    # call the calculation
    out1, out2, out3 = calc_stuff(parameter = parameter)

    # put results into correct output list
    output1.append(out1)
    output2.append(out2)
    output3.append(out3)

我知道如何在Python中启动单个线程,但我不知道如何“收集”结果。

多个进程也可以——在这种情况下,只要是最简单的就行。我目前使用的是Linux,但代码应该在Windows和Mac上运行。

并行化这段代码最简单的方法是什么?


当前回答

tqdm库的并发包装器是并行化长时间运行代码的好方法。tqdm通过智能进度表提供当前进度和剩余时间的反馈,我发现这对于长时间计算非常有用。

通过对thread_map的简单调用,循环可以被重写为并发线程,或者通过对process_map的简单调用,循环可以被重写为并发多进程:

from tqdm.contrib.concurrent import thread_map, process_map


def calc_stuff(num, multiplier):
    import time

    time.sleep(1)

    return num, num * multiplier


if __name__ == "__main__":

    # let's parallelize this for loop:
    # results = [calc_stuff(i, 2) for i in range(64)]

    loop_idx = range(64)
    multiplier = [2] * len(loop_idx)

    # either with threading:
    results_threading = thread_map(calc_stuff, loop_idx, multiplier)

    # or with multi-processing:
    results_processes = process_map(calc_stuff, loop_idx, multiplier)

其他回答

假设我们有一个async函数

async def work_async(self, student_name: str, code: str, loop):
"""
Some async function
"""
    # Do some async procesing    

这需要在一个大数组上运行。有些属性被传递给程序,有些则来自数组中字典元素的属性。

async def process_students(self, student_name: str, loop):
    market = sys.argv[2]
    subjects = [...] #Some large array
    batchsize = 5
    for i in range(0, len(subjects), batchsize):
        batch = subjects[i:i+batchsize]
        await asyncio.gather(*(self.work_async(student_name,
                                           sub['Code'],
                                           loop)
                       for sub in batch))

Dask期货;我很惊讶至今还没有人提起这件事……

from dask.distributed import Client

client = Client(n_workers=8) # In this example I have 8 cores and processes (can also use threads if desired)

def my_function(i):
    output = <code to execute in the for loop here>
    return output

futures = []

for i in <whatever you want to loop across here>:
    future = client.submit(my_function, i)
    futures.append(future)

results = client.gather(futures)
client.close()

由于@iuryxavier

from multiprocessing import Pool
from multiprocessing import cpu_count


def add_1(x):
    return x + 1

if __name__ == "__main__":
    pool = Pool(cpu_count())
    results = pool.map(add_1, range(10**12))
    pool.close()  # 'TERM'
    pool.join()   # 'KILL'

由于全局解释器锁(GIL)的存在,在CPython上使用多线程并不能为纯python代码提供更好的性能。我建议使用multiprocessing模块:

pool = multiprocessing.Pool(4)
out1, out2, out3 = zip(*pool.map(calc_stuff, range(0, 10 * offset, offset)))

注意,这在交互式解释器中不起作用。

为了避免GIL周围常见的FUD:在本例中使用线程没有任何优势。这里要使用进程,而不是线程,因为它们避免了一大堆问题。

为什么不用线程和一个互斥来保护一个全局列表呢?

import os
import re
import time
import sys
import thread

from threading import Thread

class thread_it(Thread):
    def __init__ (self,param):
        Thread.__init__(self)
        self.param = param
    def run(self):
        mutex.acquire()
        output.append(calc_stuff(self.param))
        mutex.release()   


threads = []
output = []
mutex = thread.allocate_lock()

for j in range(0, 10):
    current = thread_it(j * offset)
    threads.append(current)
    current.start()

for t in threads:
    t.join()

#here you have output list filled with data

请记住,您的速度将与最慢的线程一样快