如何将仅关键字 arguments 应用于多处理池中的 function？

Question

我有一个 function 需要一个仅关键字参数并希望在进程池中运行它。 如何在过程中将我的条目从可迭代传递到 function 作为关键字参数？

import multiprocessing

greetees = ('Foo', 'Bar')

def greet(*, greetee):
    return f'Hello, {greetee}!'

我尝试使用 multiprocessing.map：

greetings = multiprocessing.Pool(2).map(greet, greetees)
for greeting in greetings:
    print(greeting)

但这引发了一个例外，正如预期的那样：

multiprocessing.pool.RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 119, in worker
    result = (True, func(*args, **kwds))
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 44, in mapstar
    return list(map(*args))
TypeError: greet() takes 0 positional arguments but 1 was given
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/bengt/Projekte/gitlab.com/PFASDR/PFASDR.Code.Main/pfasdr/neural/multi_pool_kwargs.py", line 10, in <module>
    greetings = multiprocessing.Pool(2).map(greet, greetees)
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 266, in map
    return self._map_async(func, iterable, mapstar, chunksize).get()
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 644, in get
    raise self._value
TypeError: greet() takes 0 positional arguments but 1 was given

如果我删除星号以不要求 arguments 仅是关键字，则效果很好：

[...]
def greet(greetee):
    return f'Hello, {greetee}!'
[...]

Output：

Hello, Foo!
Hello, Bar!

Answer 1

这里的一个解决方案是使用Pool.apply或Pool.apply_async ：

greetings = list(
    multiprocessing.Pool(2).apply(greet, kwds={'greetee': greetees[i]})
    for i in range(len(greetees))
)
for greeting in greetings:
    print(greeting)

Output：

Hello, Foo!
Hello, Bar!

Answer 2

感谢Mad Physicist和此 QnA ，可以使用functools.partial将仅关键字 arguments 注入 function：

from functools import partial
greetings = []
for i in range(len(greetees)):
    kwargs = {'greetee': greetees[i]}
    greet_partial = partial(greet, **kwargs)
    greetings.append(multiprocessing.Pool(2).apply(greet_partial))

或具有较少可变出血：

from functools import partial
greetings = [
    multiprocessing.Pool(2).apply(
        partial(greet, **{'greetee': greetees[i]})
    )
    for i in range(len(greetees))
]