可迭代對(duì)象去重（保持順序不變）

def filter_multi(items,key=None): ''' 可迭代對(duì)象去重(保持順序不變) [1,4,7,2,4,7,3,5] ==> [1,4,7,2,3,5] ''' its = list() for x in items: val = x if key is None else key(x) if val not in its: yield val its.append(val)

#如:print list(filter_multi([1,3,5,3,7,2,4,2])) items = [{’a’:1,’b’:2},{’a’:3,’b’:4},{’a’:1,’b’:2},{’a’:5,’b’:6}]print list(filter_multi(items,key=lambda k:(k[’a’],k[’b’])))

補(bǔ)充知識(shí)：Python特性學(xué)習(xí)——可迭代對(duì)象，迭代器(重新修正)

以前學(xué)習(xí)python都是馬馬虎虎，導(dǎo)致很多特性只是知道完全不會(huì)用，現(xiàn)在將他們重新學(xué)習(xí)

可迭代對(duì)象（Iterable）

簡(jiǎn)單來(lái)說(shuō)，所有可以放入for循環(huán)中的對(duì)象都是可迭代對(duì)象，如列表，元組，字符串，字典…

如何判斷對(duì)象是否是可迭代對(duì)象？

實(shí)際上，只要實(shí)現(xiàn)了__iter__方法的對(duì)象就是可迭代對(duì)象，這個(gè)方法用來(lái)返回迭代器本身（特別重要）。

eg:

>>> s = 'dasda'>>> s.__iter__()<str_iterator object at 0x7f23ebc44470>

python提供了方法判斷是否是可迭代對(duì)象。

>>> from collections import Iterable>>> isinstance(s,Iterable)True

迭代器（Iterator）

似乎和上面的概念很相似。實(shí)際上，所有實(shí)現(xiàn)了__next__()方法的對(duì)象都是迭代器。所有實(shí)現(xiàn)了__next__()和__iter__()方法的對(duì)象都是迭代器，所以，所有的迭代器都能放入for循環(huán)。

python中原生的迭代器不多，可以使用iter()將可迭代對(duì)象生成迭代器。

eg:

>>> s = [1,2,3,4,5]>>> s.__next__()Traceback (most recent call last): File '<stdin>', line 1, in <module>AttributeError: ’list’ object has no attribute ’__next__’>>> s = iter(s)>>> type(s)<class ’list_iterator’>>>> s.__next__()1>>> from collections import Iterator>>> isinstance(s,Iterator)True

以及迭代器的判斷方法。

做一些區(qū)分

#coding=utf-8from collections import Iterable,Iteratorclass A:#只有__next__方法。不是迭代器也不是可迭代對(duì)象 def __init__(self,start,end): self.start = start self.end = end def __next__(self): if self.start < self.end: i = self.start self.start += 1 return i else: raise StopIteration()class B:#只有__iter__方法，__iter__返回的是一個(gè)沒(méi)有__next__的對(duì)象。是可迭代對(duì)象，不是迭代器 def __init__(self,start,end): self.start = start self.end = end def __iter__(self): return selfclass C:#只有__iter__方法，__iter__返回的是一個(gè)有__next__的對(duì)象。是可迭代對(duì)象，不是迭代器 def __init__(self,start,end): self.start = start self.end = end def __iter__(self): return A(self.start,self.end)class D:#既有__iter__又有__next__，__iter__返回的是自身，有__next__的對(duì)象，是迭代器和可迭代對(duì)象 def __init__(self,start,end): self.start = start self.end = end def __iter__(self): return self def __next__(self): if self.start < self.end: i = self.start self.start += 1 return i else: raise StopIteration()class E:#既有__iter__又有__next__，__iter__返回的不是自身，有__next__的對(duì)象，是迭代器和可迭代對(duì)象 def __init__(self,start,end): self.start = start self.end = end def __iter__(self): return A(self.start,self.end) def __next__(self): if self.start < self.end: i = self.start self.start += 1 return i else: raise StopIteration()class F:#既有__iter__又有__next__，__iter__返回的是沒(méi)有__next__的對(duì)象，是迭代器和可迭代對(duì)象 def __init__(self,start,end): self.start = start self.end = end def __iter__(self): return 1 def __next__(self): if self.start < self.end: i = self.start self.start += 1 return i else: raise StopIteration()s = A(5,10)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))s = B(5,10)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))s = C(5,10)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))s = D(5,10)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))s = E(5,10)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))s = F(5,10)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))

運(yùn)行結(jié)果

Iterable: FalseIterator: FalseIterable: TrueIterator: FalseIterable: TrueIterator: FalseIterable: TrueIterator: TrueIterable: TrueIterator: TrueIterable: TrueIterator: True

for循環(huán)

很明顯看出,list是一個(gè)可迭代對(duì)象，它能放到for循環(huán)里。但list不是迭代器，把它變成迭代器后，也能放入for循環(huán)中。那么問(wèn)題來(lái)了：

for循環(huán)如何處理迭代器和可迭代對(duì)象的呢？

先來(lái)試試A-F都能不能用for

s = A(1,4)for i in s: print(i)->Traceback (most recent call last): File 'IteratorZZ.py', line 68, in <module> for i in s:TypeError: ’A’ object is not iterable#提示并非一個(gè)可迭代對(duì)象

s = B(1,4)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))for i in s: print(i)->Iterable: TrueIterator: FalseTraceback (most recent call last): File 'IteratorZZ.py', line 75, in <module> for i in s:TypeError: iter() returned non-iterator of type ’B’#提示__iter__()返回的不是一個(gè)迭代器

s = C(1,4)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))for i in s: print(i)->Iterable: TrueIterator: False123#成功

s = D(1,4)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))for i in s: print(i)->Iterable: TrueIterator: True123#成功

s = E(1,4)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))for i in s: print(i)->Iterable: TrueIterator: True123#成功

s = F(1,4)print(’Iterable:’,isinstance(s,Iterable))print(’Iterator:’,isinstance(s,Iterator))for i in s: print(i)->Iterable: TrueIterator: TrueTraceback (most recent call last): File 'IteratorZZ.py', line 115, in <module> for i in s:TypeError: iter() returned non-iterator of type ’int’#失敗，__iter__返回的不是迭代器

由此可見(jiàn)，for只能作用在可迭代對(duì)象上(注意，Iterable和Iterator不沖突，一個(gè)對(duì)象即可以是Iterable也可以是Iterator)。并且，這個(gè)可迭代對(duì)象的__iter__返回的只需要是一個(gè)有__next__的對(duì)象（即便它不是迭代器，如C類，__iter__返回的是并非迭代器的A類）。

所以for的工作流程：

1. 是否有__iter__，沒(méi)有則出錯(cuò)

2. 調(diào)用__iter__

3. 返回的對(duì)象不斷next()直到StopIteration

總結(jié)

可迭代對(duì)象只需有__iter__方法，并且不限制它非得返回有__next__的對(duì)象

迭代器必須同時(shí)擁有__iter__和__next__，并且__iter__返回的對(duì)象不一定有__next__方法（F類）。

for循環(huán)可以作用在可迭代對(duì)象上。成功的for必須是__iter__返回有__next__方法的對(duì)象。

疑問(wèn)

迭代器必須同時(shí)實(shí)現(xiàn)__next__和__iter__,那non-iterator是不是說(shuō)的是非迭代器呢？但是E類的__iter__返回的對(duì)象(A)不是迭代器但也能for，這該怎么解釋呢？

回答

Python里有一個(gè)原則，鴨子類型，即只要一個(gè)生物長(zhǎng)得像鴨子，就認(rèn)為它是鴨子。

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