当我在 Jupyter notebook (python3.5) 中运行以下代码时:
def process(doc):
print('Process Hey!')
with Pool() as pool:
results = pool.imap(process, docs)
这不会给笔记本打印，但如果我尝试改用 pool.map，它会吐出文字。所以我真的很好奇这里发生了什么。谢谢!
请您参考如下方法:
来自文档 imap map() 的懒惰版本，imap 返回一个迭代器 map 返回一个列表:
In [24]: with Pool() as pool:
results = pool.map(process, docs)
print(type(results))
....:
Process Hey!
Process Hey!
<class 'list'>
In [25]: with Pool() as pool:
results = pool.imap(process, docs)
print(type(results))
....:
<class 'multiprocessing.pool.IMapIterator'>
In [27]: with Pool() as pool:
results = pool.imap(process, docs)
for _ in results:
pass
....:
Process Hey!
Process Hey!
In [28]: with Pool() as pool:
results = pool.imap(process, docs)
list(results)
....:
Process Hey!
Process Hey!
相当于python2中itertools.imap和map的区别，懒惰求值vs贪婪求值:
In [3]: from itertools import imap
In [4]: r = imap(lambda x: x+1, [1,2,3])
In [4]: r
Out[5]: <itertools.imap at 0x7f09170f1e10>
In [6]: list(r)
Out[6]: [2, 3, 4]
In [7]: r = map(lambda x: x+1, [1,2,3])
In [8]: r
Out[8]: [2, 3, 4]
