python内置函数简单谈论

在编写python程序时或工作脚本时，需要完成某个功能，可以选择编写一个具体的函数达到目的，当然也可以通过匿名/Python 内建函数来完成。本问讲述常见的Python匿名、内建函数---lambda,map,filter,reduce .

实例说明

2.1 Lambda 是一个匿名函数，
其语法为:lambda parameters:express
parameters：可选,通常是逗号分隔的变量表达式形式，即位置参数。
expression：不能包含分支或循环（但允许条件表达式），也不能包含return（或yield）函数。如果为元组，则应用圆括号将其包含起来。
调用lambda函数，返回的结果是对表达式计算产生的结果。
根据参数是否为2的倍数 返回yes or no

1. In [21]: s = lambda x:"yes" if x%2 == 0 else "no"
2. In [22]: s(3)
3. Out[22]: 'no'
4. In [24]: s(4)
5. Out[24]: 'yes'

上面例子中，将if...else语句缩减为单一的条件表达式，语法为：

1. expression1 if A else expression2

如果A为True，条件表达式的结果为expression1，否则为expression2 。
使用匿名函数的优点是代码简洁 ，但是不宜读 ，推荐 for ..in ...if  语法进行遍历迭代。

2.2 filter 过滤
其语法为 filter(function, sequence)
意义是:对sequence中的item 依次迭代调用function(item)，根据执行结果为True或者False来保留或者丢弃该元素。并将保留的item组成一个List/String/Tuple（取决于sequence的类型 ）
filter()把传入的函数依次作用于每个元素，然后根据返回值是True还是False决定保留还是丢弃该元素。

1. In [15]: filter(lambda x:True if x % 3 == 0 else False, range(10))
2. Out[15]: [0, 3, 6, 9] #返回list
3. In [17]: filter(lambda x:True if x % 3 == 0 else False, (1,2,3,5,6,7,8,9,10))
4. Out[17]: (3, 6, 9) #返回Tuple 元组

2.3 map 映射
其语法为map(function, sequence):
map()的用法和filter()类似，也是将序列放入函数进行运算，但是，不论运算结果为什么，map()将返回结果也即返回的结果元素个数和sequence一样多，filter是返回符合条件的值。这是map()和filter()的主要区别。请注意，filter()和map()中的function都必要有一个返回值。同样获取能否被3整除的值,map 返回的是True/False 列表，filter返回的是符合条件的元素结果集。

1. In [18]: list(map(lambda x:True if x % 3 == 0 else False, range(10)))
2. Out[18]: [True, False, False, True, False, False, True, False, False, True]

2.4 reduce
其语法为 reduce(function, sequence, starting_value)
意义是:对sequence中的item 依次调用function。reduce(f, [x1, x2, x3, x4]) = f(f(f(x1, x2), x3), x4)
如果有starting_value，还可以作为初始值调用，比如1-100的list求和:

1. In [11]: def add(x,y):
2.     ...: return x + y
3.     ...:
4. In [12]: reduce(add, range(1, 100))
5. Out[12]: 4950
6. In [13]: reduce(add, range(1, 10))
7. Out[13]: 45
8. # 带上 starting_value 参数，相当于 1+2+3+4+5+6+7+8++9+10+20
9. In [14]: reduce(add, range(1, 10),20)
10. Out[14]: 65

三 小结
   本文总结了python内置函数和匿名函数，在编程时合理的运用能够极大的提高开发效率。
  推荐 https://www.python.org/dev/peps/pep-0289/

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