Section 26.1: 过滤器的基本用途

根据某些标准过滤掉序列中的元素：

names = ['Fred', 'Wilma', 'Barney']

def long_name(name):
  return len(name) > 5

filter(long_name, names) # returns a generator
# Out: 

list(filter(long_name, names)) # cast to list
# Out: ['Barney']

(name for name in names if len(name) > 5) # equivalent generator expression
# Out:  at 0x000001C6F49BF4C0>

Section 26.2: 不带函数的过滤器

如果函数参数为 “None”，则将使用标识函数：

list(filter(None, [1, 0, 2, [], '', 'a'])) # discards 0, [] and ''
# Out: [1, 2, 'a']

(i for i in [1, 0, 2, [], '', 'a'] if i) # equivalent generator expression

Section 26.3: 作为短路检查的过滤器

filter（python 3.x）和 ifilter（python 2.x）会返回一个生成器，因此在创建 or 或 and 等短路测试时非常方便：

找出第一个小于 100 的元素：

car_shop = [('Toyota', 1000), ('rectangular tire', 80), ('Porsche', 5000)]
def find_something_smaller_than(name_value_tuple):
  print('Check {0}, {1}$'.format(*name_value_tuple))
  return name_value_tuple[1] < 100
next(filter(find_something_smaller_than, car_shop))

# Print: Check Toyota, 1000$
# Check rectangular tire, 80$
# Out: ('rectangular tire', 80)

next-function 给出了下一个（这里是第一个）元素，因此是短路的原因。

Section 26.4: 互补函数：filterfalse、ifilterfalse

在 itertools 模块中有一个过滤器的补充功能：

from itertools import filterfalse

它的工作原理与生成器过滤器完全相同，但只保留 False 元素：

# Usage without function (None):
list(filterfalse(None, [1, 0, 2, [], '', 'a'])) # discards 1, 2, 'a'
# Out: [0, [], '']

# Usage with function
names = ['Fred', 'Wilma', 'Barney']

def long_name(name):
  return len(name) > 5

list(filterfalse(long_name, names))
# Out: ['Fred', 'Wilma']

# Short-circuit usage with next:
car_shop = [('Toyota', 1000), ('rectangular tire', 80), ('Porsche', 5000)]
def find_something_smaller_than(name_value_tuple):
  print('Check {0}, {1}$'.format(*name_value_tuple))
  return name_value_tuple[1] < 100
next(filterfalse(find_something_smaller_than, car_shop))
# Print: Check Toyota, 1000$
# Out: ('Toyota', 1000)

# Using an equivalent generator:
car_shop = [('Toyota', 1000), ('rectangular tire', 80), ('Porsche', 5000)]
generator = (car for car in car_shop if not car[1] < 100)
next(generator)