如何使用Gevent并行执行带参任务并获取返回结果？

我有一段基于gevent的并行代码，原本能正常运行并打印计算结果，耗时约1秒：

from gevent import monkey; monkey.patch_all()
from gevent.pool import Pool
import time

def work(x):
  time.sleep(1)  # 可替换为IO相关操作，比如requests.get()
  newVal = x ** 2
  print(x, newVal)

start = time.perf_counter()
p = Pool()
l = [x for x in range(10)]
for i,x in enumerate(l):
  p.spawn(work, x)
p.join()
print(round(time.perf_counter() - start, 3))  # 输出约1.001或1.002

运行输出：

(test) C:\DEV\Python-Diverses\gevent>python try.py
0 0
1 1
2 4
3 9
4 16
5 25
6 36
7 49
8 64
9 81
1.036

我希望从work函数返回计算结果并收集所有值，于是修改了代码：

from gevent import monkey; monkey.patch_all()
from gevent.pool import Pool
import time

def work(x):
  time.sleep(1)  # 可替换为IO相关操作，比如requests.get()
  newVal = x ** 2
  return (newVal)

start = time.perf_counter()
p = Pool()
l = [x for x in range(10)]
ergList = []
for i,x in enumerate(l):
  erg = p.spawn(work, x)
  ergList.append(erg)

p.join()
print(ergList)
print(round(time.perf_counter() - start, 3))  # 输出约1.001或1.002

运行耗时仍为1秒，但输出的是Greenlet实例而非计算结果：

(test) C:\DEV\Python-Diverses\gevent>python try.py
[<Greenlet at 0x180704a55e0: _run>, <Greenlet at 0x18070533f40: _run>, <Greenlet at 0x18070563180: _run>, <Greenlet at 0x180705632c0: _run>, <Greenlet at 0x18070563400: _run>, <Greenlet at 0x18070563540: _run>, <Greenlet at 0x18070563680: _run>, <Greenlet at 0x180705637c0: _run>, <Greenlet at 0x18070563900: _run>, <Greenlet at 0x18070563a40: _run>]
1.07

请问如何在并行执行任务的同时获取返回的计算结果，而非Greenlet实例？

方法1：调用Greenlet实例的get()方法 #

p.spawn()返回的是Greenlet对象，调用它的get()方法可以获取函数的返回值。需在p.join()之后调用，确保所有任务执行完毕：

from gevent import monkey; monkey.patch_all()
from gevent.pool import Pool
import time

def work(x):
  time.sleep(1)
  newVal = x ** 2
  return newVal

start = time.perf_counter()
p = Pool()
l = [x for x in range(10)]
ergList = []
for x in l:
  erg = p.spawn(work, x)
  ergList.append(erg)

p.join()
# 遍历Greenlet列表，提取返回结果
results = [g.get() for g in ergList]
print(results)
print(round(time.perf_counter() - start, 3))

运行输出：

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
1.05

方法2：使用Pool.map()简化代码 #

gevent的Pool提供了map()方法，与Python内置map功能类似，但会并行执行任务，直接返回结果列表，无需手动处理Greenlet对象：

from gevent import monkey; monkey.patch_all()
from gevent.pool import Pool
import time

def work(x):
  time.sleep(1)
  newVal = x ** 2
  return newVal

start = time.perf_counter()
p = Pool()
l = [x for x in range(10)]
# 并行执行任务并直接获取结果
results = p.map(work, l)
print(results)
print(round(time.perf_counter() - start, 3))

运行输出与方法1一致，代码更简洁直观。

内容的提问来源于stack exchange，提问作者Rapid1898