python之map函数介绍使用

   Python 内置了很多非常有用的函数 比如map() ,reduce(),filter(),还有lambda。熟练应用这些函数可以在写python程序的时候构建精简的代码。本文先来了解map函数。

用法：

1. map(func, seq1[, seq2,…])

map接收两个参数，第一个参数是函数名，第二个是一个或多个可迭代的序列，返回的是一个集合。运行时，map()将func作用于序列中的每一个元素，并将结果作为一个list返回。如果func为None，作用同zip()。
2.1 当seq 只有一个时，map函数返回将func函数作用于 seq每个元素并返回一个新的list集合,

比如需要将seq的元素乘以2 ,使用map可以写成

1. In [4]: l=[1, 2, 3, 4, 5, 6, 7, 8, 9]
2. In [5]: map(lambda x:x*2 ,l)
3. Out[5]: [2, 4, 6, 8, 10, 12, 14, 16, 18]

如果使用函数来做:

1. rest=[]
2. for i in l:
3.   rest.append(i*2)
4. print rest

map作为高阶函数，能够把运算规则抽象化,显然map 更精简。

2.2 当有多个seq时，map可以并行的取每个seq对应的第M个元素 seqN[M]，进行计算。

例子:

1. In [9]: map(lambda x , y : x * y, [2,4,6],[3,2,1])
2. Out[9]: [6, 8, 6]

记得多个seq的元素个数必须一致 不然会报错 

1. In [1]: print map(lambda x , y : x ** y, [2,4,6],[3,2,1])
2. [8, 16, 6]
3. seq1=[2,4,6] ,seq2=[3,2] 元素个数不一致则报错。
4. In [3]: print map(lambda x , y : x ** y, [2,4,6],[3,2])
5. ---------------------------------------------------------------------------
6. TypeError Traceback (most recent call last)
7. <ipython-input-3-d075c46afd0b> in <module>()
8. ----> 1 print map(lambda x , y : x ** y, [2,4,6],[3,2])
9. <ipython-input-3-d075c46afd0b> in <lambda>(x, y)
10. ----> 1 print map(lambda x , y : x ** y, [2,4,6],[3,2])
11. TypeError: unsupported operand type(s) for ** or pow(): 'int' and 'NoneType'
12. In [4]:

2.3 map在多进程中的应用。

1. #!/usr/bin/env python
2. # encoding: utf-8
3. """
4. author: [email protected]
5. time: 2017/7/16 上午10:42
6. func: 参考网络上的例子 
7. """
8. import urllib2
9. from multiprocessing.dummy import Pool as ThreadPool
10. urls = [
11.         'http://www.python.org',
12.         'http://www.python.org/about/',
13.         'http://www.onlamp.com/pub/a/python/2003/04/17/metaclasses.html',
14.         'http://www.python.org/doc/',
15.         'http://www.python.org/download/',
16.         'http://www.python.org/getit/',
17.         'http://www.python.org/community/',
18.         'https://wiki.python.org/moin/',
19.         'http://planet.python.org/',
20.         'https://wiki.python.org/moin/LocalUserGroups',
21.         'http://www.python.org/psf/',
22.         'http://docs.python.org/devguide/',
23.         'http://www.python.org/community/awards/'
24.         ]
26. # Make the Pool of workers
27. pool = ThreadPool(4)
28. # Open the urls in their own threads
29. # and return the results
30. results = pool.map(urllib2.urlopen, urls)
31. #close the pool and wait for the work to finish
32. pool.close()
33. pool.join()

该例子使用了multiprocessing.dummy的线程池Pool的方式，利用用map的特性来处理url集合。
参考文档
[1] [Python] Python中的一些特殊函数

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