使用 multiprocessing 模块中的 Process 类创建进程：
from multiprocessing import Process
创建进程与创建线程的语法十分相似。
多进程的问题： 进程之间不能共享资源，例如下面的例子，如果process2 是 ask_user 的进程，则会出现错误 EOFError: EOF when reading a line
process2 = Process(target=ask_user) # Will cause EOFError!!
因为 process2 无法从终端读取数据。
import time
from multiprocessing import Process
def ask_user():
start = time.time()
user_input = input("Enter your name: ")
greet = f"Hello, {user_input}"
print(greet)
print(f'ask_user, {time.time() - start}')
def complex_calculation():
start = time.time()
print('Started calculating...')
[x**2 for x in range(20000000)]
print(f'complex_calculation, {time.time() - start}')
# for windows: put my_process.start(), my_process.join()
# in this if statement is important
# or new processes will continuously start
if __name__ == '__main__':
start = time.time()
ask_user()
complex_calculation()
print(f'Single thread total time: {time.time() - start}')
process = Process(target=complex_calculation)
# if here target = ask_user, input cannot be read
# user_input = input("Enter your name: ")
# EOFError: EOF when reading a line
# because resources cannot be shared among processes
process2 = Process(target=complex_calculation)
process.start()
process2.start()
start = time.time()
process.join()
process2.join()
print(f'Two processes total time: {time.time() - start}')
进程池
process pool 与 thread pool 的语法十分相似：
import time
from concurrent.futures import ProcessPoolExecutor
def ask_user():
start = time.time()
user_input = input("Enter your name: ")
greet = f"Hello, {user_input}"
print(greet)
print(f'ask_user, {time.time() - start}')
def complex_calculation():
start = time.time()
print('Started calculating...')
[x**2 for x in range(20000000)]
print(f'complex_calculation, {time.time() - start}')
if __name__ == '__main__':
start = time.time()
complex_calculation()
complex_calculation()
print(f'Single thread total time: {time.time() - start}')
start = time.time()
with ProcessPoolExecutor(max_workers=2) as pool:
pool.submit(complex_calculation)
pool.submit(complex_calculation)
print(f'Two processes total time: {time.time() - start}')
输出：
Started calculating... complex_calculation, 3.7751059532165527 Started calculating... complex_calculation, 3.809253215789795 Single thread total time: 7.584359169006348 Started calculating... Started calculating... complex_calculation, 3.9134361743927 complex_calculation, 3.94466495513916 Two processes total time: 4.0857954025268555
进程池的写法比创建进程简洁。
使用进程，程序运行时间明显缩短。两次调用 complex_calculation()， 如果不使用两个进程，调用时间累加，需要 7.5 秒，使用两个进程单独完成计算，则所需时间缩短为 4 秒。
一般情况下，如果是有需要等待的多任务程序，使用多线程。
如果 CPU 多核，并且需要执行多项使用CPU的复杂计算，使用多进程。
