Python迭代和迭代器

1、我們將要來學(xué)習(xí)python的重要概念迭代和迭代器，通過簡(jiǎn)單實(shí)用的例子如列表迭代器和xrange。可迭代一個(gè)對(duì)象，物理或者虛擬存儲(chǔ)的序列。list，tuple，strins，dicttionary，set以及生成器對(duì)象都是可迭代的，整型數(shù)是不可迭代的。如果你不確定哪個(gè)可迭代哪個(gè)不可以，你需要用python內(nèi)建的iter()來幫忙。>>> iter(1,2,3)<listiterator object at 0x026C8970>>>> iter(1:2, 2:4)<dictionary-keyiterator object at 0x026C

2、C1B0>>>> iter(1234)Traceback (most recent call last): File "<pyshell#145>", line 1, in <module> iter(1234)TypeError: 'int' object is not iterableiter()為list返回了listiterator對(duì)象，為dictionary返回了dictionary-keyiterator對(duì)象。類似對(duì)其他可迭代類型也會(huì)返回迭代器對(duì)象。iter()用在自定義的類型會(huì)怎樣呢？我們先自己定

3、義一個(gè)String類：class String(object): def _init_(self, val): self.val = val def _str_(self): return self.valst = String('sample string')那么，st是可迭代的嗎？>>> iter(st)TypeError: 'String' object is not iterable你可能會(huì)有幾個(gè)問題要問：· 怎么讓自定義的類型可迭代？· iter()究竟做了些什么？讓我們補(bǔ)充String類來找找答案class St

4、ring(object): def _init_(self, val): self.val = val def _str_(self): return self.val def _iter_(self): print "This is _iter_ method of String class" return iter(self.val) #self.val is python string so iter() will return it's iterator>>> st = String('Sample String')>

5、>> iter(st)This is _iter_ method of String class<iterator object at 0x026C8150>在String類中需要一個(gè)_iter_方法把String類型變成可迭代的，這就是說iter內(nèi)部調(diào)用了iterable._iter_()別急，不是只有增加_iter()方法這一種途徑class String(object): def _init_(self, val): self.val = val def _str_(self): return self.val def _getitem_(self, index)

6、: return self.valindex>>> st = String('Sample String')>>> iter(st)<iterator object at 0x0273AC10>itr也會(huì)調(diào)用iterable._getitem_()，所以我們用_getitem_方法讓String類型可迭代。如果在String類中同時(shí)使用_iter_()和_getitem_(),就只有_iter_會(huì)起作用。自動(dòng)迭代for循環(huán)會(huì)自動(dòng)迭代for x in iterable: print x我們可以不用for循環(huán)來實(shí)現(xiàn)嗎？def iter

7、ate_while(iterable): index = 0 while(i< len(iterable): print iterablei i +=1這樣做對(duì)list和string是管用的，但對(duì)dictionary不會(huì)奏效，所以這絕對(duì)不是python式的迭代，也肯定不能模擬for循環(huán)的功能。我們先看迭代器，等下回再過頭來。迭代器關(guān)于迭代器先說幾條.· 1. 迭代器對(duì)象在迭代過程中會(huì)會(huì)產(chǎn)生可迭代的值，next()或者_(dá)next()_是迭代器用來產(chǎn)生下一個(gè)值的方法。· 2. 它會(huì)在迭代結(jié)束后發(fā)出StopIteration異常。· 3. iter()函數(shù)返回迭代

8、器對(duì)象· 4. 如果iter()函數(shù)被用在迭代器對(duì)象，它會(huì)返回對(duì)象本身我們?cè)囈辉嚹７耭or循環(huán)def simulate_for_loop(iterable): it = iter(iterable) while(True):try: print next(it)except StopIteration: break>>> simulate_for_loop(23,12,34,56)23123456前面我們看過了iterable類，我們知道iter會(huì)返回迭代器對(duì)象?，F(xiàn)在我們?cè)囍斫獾黝惖脑O(shè)計(jì)。class Iterator: def _init_(self, ite

9、rable) self.iterable = iterable . . def _iter_(self): #iter should return self if called on iterator return self def next(self): #Use _next_() in python 3.x if condition: #it should raise StopIteration exception if no next element is left to return raise StopIteration我們學(xué)了夠多的迭代和迭代器，在python程序中不會(huì)用到比這更深

10、的了。但是為了學(xué)習(xí)的目的我們就到這兒。列表迭代器你可能會(huì)在面試中寫這個(gè)，所以打起精神來注意了class list_iter(object): def _init_(self, list_data): self.list_data = list_data self.index = 0 def _iter_(self): return self def next(self): #Use _next_ in python 3.x if self.index < len(self.list_data): val = self.list_dataself.index self.index += 1

11、 return val else: raise StopIteration()我們來用list_iter自己定義一個(gè)列表迭代器class List(object): def _init_(self, val): self.val = val def _iter_(self): return list_iter(self.val)>>> ls = List(1,2,34)>>> it = iter(ls)>>> next(it)1>>> next(it)2>>> next(it)34>>>

12、 next(it)Traceback (most recent call last): File "<pyshell#254>", line 1, in <module> next(it) File "<pyshell#228>", line 13, in next raise StopIteration()StopIterationxrange從一個(gè)問題開始xrange是迭代還是迭代器？我們來看看>>> x = xrange(10)>>> type(x)<type '

13、xrange'>幾個(gè)關(guān)鍵點(diǎn)：· 1. iter(xrange(num)應(yīng)該被支持· 2. 如果iter(xrange(num)返回同樣的對(duì)象（xrange類型）那xrange就是迭代器· 3. 如果iter(xrange(num)返回一個(gè)迭代器對(duì)象那xrange就是迭代>>> iter(xrange(10)<rangeiterator object at 0x0264EFE0>它返回了rangeiterator，所以我們完全可以叫它迭代器。讓我們用最少的xrange函數(shù)實(shí)現(xiàn)自己的xrangexrange_iteratorclass xrange_iter(object): def _init_(self, num): self.num = num self.start = 0 def _iter_(self): return self def next(self): if self.start < self.num: val = self.start self.start += 1 return val else: raise Sto