python中内置函数`map`

Return an iterator that applies function to every item of iterable, yielding the results.

1	a = map(fun, iterable)

a是一个迭代器，是惰性的；换句话说就是，执行上面一句代码，fun函数并没有执行，只有调用next(a)时，才会执行fun函数（体现了迭代器的特点：按需获取）。

验证的例子如下:

>>> def fun(value):
...     print('I have be run %d' % value)
...     return value
...
>>> a = map(fun, range(5))
>>> next(a)
I have be run 0
0
>>> next(a)
I have be run 1
1
>>>

生成器原理和这差不多。

concurrent.futures中`map`函数

Equivalent to map(func, *iterables) except func is executed asynchronously and several calls to func may be made concurrently.

官方文档说和内置map函数差不多，只不过函数func可能被同时调用。但实际情况是有一点差别的，下面进行说明。

1	a = executor.map(func, *iterables, timeout=None, chunksize=1)

a是一个生成器，是惰性的，但是执行上面一行代码后，func函数执行了（注意和内置map函数区别）；这个返回的生成器只是针对函数func返回的结果而言的。
验证的例子如下：

from concurrent.futures import ThreadPoolExecutor
import threading
import  time
def task(name):
    print("name",name)
    time.sleep(1)
    return 1

if __name__ == "__main__":
    start = time.time()
    ex = ThreadPoolExecutor(5)
    res = ex.map(task,range(5))
    #print(next(res))
    ex.shutdown(wait=True)
    #print(list(res))

    print("main")
    end = time.time()
    print(end - start)

结果：

>>> python3 test.py
name 0
name 1
name 2
name 3
name 4
main
0.002694368362426758

为进一步探究其原因，看一下concurrent.futures中map函数的源码。
模块的位置如下：

>>> import concurrent.futures
>>> print(concurrent.futures.__file__)
/usr/lib/python3.5/concurrent/futures/__init__.py
>>>

对应map源码的位置/usr/lib/python3.5/concurrent/futures/_base.py

def map(self, fn, *iterables, timeout=None, chunksize=1):
    if timeout is not None:
        end_time = timeout + time.time()

    fs = [self.submit(fn, *args) for args in zip(*iterables)]

    # Yield must be hidden in closure so that the futures are submitted
    # before the first iterator value is required.
    def result_iterator():
        try:
            for future in fs:
                if timeout is None:
                    yield future.result()
                else:
                    yield future.result(end_time - time.time())
        finally:
            for future in fs:
                future.cancel()
    return result_iterator()

根据源码可知，第5行列表推到，函数fun已经被调用，最后返回是fun函数调用结果的生成器。和上面的讨论一致。

结束。

python中内置函数map

concurrent.futures中map函数

python中内置函数`map`

concurrent.futures中`map`函数