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Python 函数编程

functools, itertools, operator是Python标准库为我们提供的支持函数式编程的三大模块

1 functools

functools 提供了一些非常有用的高阶函数。

1.1 partial

>>> from functools import partial
>>> basetwo = partial(int, base=2)
>>> basetwo('10010')
18

basetwo('10010')实际上等价于调用int('10010', base=2)

1.2 wraps

  • 装饰器会遗失被装饰函数的__name__和__doc__等属性，可以使用@wraps来恢复。
from functools import wraps
def my_decorator(f):
    @wraps(f)
    def wrapper():
        """wrapper_doc"""
        print('Calling decorated function')
        return f()
    return wrapper

@my_decorator
def example():
    """example_doc"""
    print('Called example function')

>>> example.__name__
'example'
>>> example.__doc__
'example_doc'

2 itertools

itertools 提供了非常有用的用于操作迭代对象的函数

2.1 无限迭代器

count 无限迭代器

  • count(start=0, step=1) 会返回一个无限的整数iterator
from itertools import count
>>> for i in zip(count(1), ['a', 'b', 'c']):
...     print i
... 
(1, 'a')
(2, 'b')
(3, 'c')

cycle 序列无限重复迭代

  • cycle(iterable)
>>> from itertools import cycle
>>> for i in zip(range(6), cycle(['a', 'b', 'c'])):
...     print i
... 
(0, 'a')
(1, 'b')
(2, 'c')
(3, 'a')
(4, 'b')
(5, 'c')

repeat 单个元素无限重复

  • repeat(object[, times])
>>> from itertools import repeat
>>> for i in zip(count(1), repeat('over-and-over', 5)):
...     print i
... 
(1, 'over-and-over')
(2, 'over-and-over')
(3, 'over-and-over')
(4, 'over-and-over')
(5, 'over-and-over')

2.2 chain

  • itertools.chain( * iterables)可以将多个iterable组合成一个iterator。
>>> from itertools import chain
>>> list(chain([1, 2, 3], ['a', 'b', 'c']))
[1, 2, 3, 'a', 'b', 'c']
  • chain.from_iterable(iterable)和chain类似
    • 但是只是接收单个iterable，然后将这个iterable中的元素组合成一个iterator。
>>> from itertools import chain
>>> list(chain.from_iterable(['ABC', 'DEF']))
['A', 'B', 'C', 'D', 'E', 'F']

2.3 compress

  • compress(data, selectors)
    • 接收两个iterable作为参数,只返回selectors中对应的元素为True的data
    • 当data/selectors之一用尽时停止
>>> from itertools import compress
>>> list(compress([1, 2, 3, 4, 5], [True, True, False, False, True]))
[1, 2, 5]

2.4 islice

  • islice(iterable, stop)
  • islice(iterable, start, stop[, step])
    • 与python切片类似， 只是不能对start、start和step使用负值。
>>> from itertools import islice
>>> for i in islice(range(40), 0, 100, 10):
...     print i
... 
0
10
20
30

2.5 tee

  • tee(iterable, n=2)
    • 返回n个独立的iterator
>>> from itertools import islice, tee
>>> r = islice(count(), 5)
>>> i1, i2 = tee(r)
# tee 调用后， r 失效

2.6 starmap

  • 和 map 类似，只是 把list 中的每个元素 作为迭代器， 迭代结果 作为函数调用参数
>>> from itertools import starmap
>>> list(starmap(os.path.join, [ ("a","b") ]))
['a/b']
>>> list(starmap(os.path.join, [ "abc" , "cdf" ]))
['a/b/c', 'c/d/f']

2.7 takewhile

  • takewhile(predicate, iterable)
    • 只要predicate返回True，不停地返回iterable中的元素
    • 知道 predicate返回False
def less_than_10(x):
    return x < 10
itertools.takewhile(less_than_10, itertools.count())
=> 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
itertools.takewhile(is_even, itertools.count())
=> 0

2.8 dropwhile

  • dropwhile(predicate, iterable)
    • 和 takewhile 相反
    • 在predicate返回True时舍弃元素，然后返回其余迭代结果
itertools.dropwhile(less_than_10, itertools.count())
=> 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, ...
itertools.dropwhile(is_even, itertools.count())
=> 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...

2.9 groupby

  • groupby(iterable, key=None)
    • 把iterator中 相邻的重复元素挑出来放在一起
>>> from itertools import groupby
>>> for key, group in itertools.groupby('AAAABBBCCDAABBB'):
...     print key , list(group)
... 
A ['A', 'A', 'A', 'A']
B ['B', 'B', 'B']
C ['C', 'C']
D ['D']
A ['A', 'A']
B ['B', 'B', 'B']

2.10 排列组合 generators

product

  • product( * iterables, repeat=1)
    • product(A, B) returns the same as ((x,y) for x in A for y in B)
    • product(A, repeat=4) means the same as product(A, A, A, A)
    • 数数中的 可重复选取 的排列
>>> from itertools import product
>>> from itertools import product
>>> list(product(range(3), repeat=2))
[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

permutations

  • permutations(iterable, r=None)
    • 返回长度为r的所有可能的排列 P(n,r)
>>> from itertools import permutations
>>> list( permutations( range(3) ) )
[(0, 1, 2), (0, 2, 1), (1, 0, 2), (1, 2, 0), (2, 0, 1), (2, 1, 0)]
>>> list( permutations( range(3) , r=2) )
[(0, 1), (0, 2), (1, 0), (1, 2), (2, 0), (2, 1)]
>>> list( permutations( range(3) , r=1) )
[(0,), (1,), (2,)]

combinations

  • combinations(iterable, r)
    • 返回长度为r的所有可能的组合 C(n,r)
>>> from itertools import combinations
>>> list( combinations( range(3) , 2) )
[(0, 1), (0, 2), (1, 2)]
>>> list( combinations( range(3) , 1) )
[(0,), (1,), (2,)]
>>> list( combinations( range(3) , 3) )
[(0, 1, 2)]

combinations_with_replacement

  • 可重复选取的 组合
>>> from itertools import combinations_with_replacement
>>> list( combinations_with_replacement( range(3) , 2) )
[(0, 0), (0, 1), (0, 2), (1, 1), (1, 2), (2, 2)]
>>> list( combinations_with_replacement( range(3) , 1) )
[(0,), (1,), (2,)]
>>> list( combinations_with_replacement( range(3) , 3) )
[(0, 0, 0), (0, 0, 1), (0, 0, 2), (0, 1, 1), (0, 1, 2), (0, 2, 2), (1, 1, 1), (1, 1, 2), (1, 2, 2), (2, 2, 2)]

3 operator

这个模块提供了一系列的函数操作。比如，operator.add(x, y)等于x+y

'abs', 'add', 'and_', 'attrgetter', 'concat', 'contains', 'countOf', 'delitem', 'delslice', 'div', 'eq', 'floordiv', 'ge', 'getitem', 'getslice', 'gt', 'iadd', 'iand', 'iconcat', 'idiv', 'ifloordiv', 'ilshift', 'imod', 'imul', 'index', 'indexOf', 'inv', 'invert', 'ior', 'ipow', 'irepeat', 'irshift', 'isCallable', 'isMappingType', 'isNumberType', 'isSequenceType', 'is_', 'is_not', 'isub', 'itemgetter', 'itruediv', 'ixor', 'le', 'lshift', 'lt', 'methodcaller', 'mod', 'mul', 'ne', 'neg', 'not_', 'or_', 'pos', 'pow', 'repeat', 'rshift', 'sequenceIncludes', 'setitem', 'setslice', 'sub', 'truediv', 'truth', 'xor'
and_(a,b)  -- Same as a & b.
iadd(a, b) -- Same as a += b.
index(a) -- Same as a.__index__()
indexOf(a, b) -- Return the first index of b in a.
inv(a) -- Same as ~a.
invert(a) -- Same as ~a.
irepeat(a, b) -- Same as a *= b, where a is a sequence, and b is an integer.
isCallable(a) -- Same as callable(a).
is_(a, b) -- Same as a is b.
is_not(a, b) -- Same as a is not b.
itruediv(a, b) -- Same as a /= b when __future__.division is in effect.
truth(a) -- Return True if a is true, False otherwise.