詳解Python之可迭代對(duì)象,迭代器和生成器

更新時(shí)間：2021年12月24日 09:33:28 作者：Python_xiaowu

這篇文章主要為大家介紹了Python之可迭代對(duì)象,迭代器和生成器，具有一定的參考價(jià)值，感興趣的小伙伴們可以參考一下，希望能夠給你帶來(lái)幫助

一、概念描述

可迭代對(duì)象就是可以迭代的對(duì)象，我們可以通過(guò)內(nèi)置的iter函數(shù)獲取其迭代器，可迭代對(duì)象內(nèi)部需要實(shí)現(xiàn)__iter__函數(shù)來(lái)返回其關(guān)聯(lián)的迭代器;

迭代器是負(fù)責(zé)具體數(shù)據(jù)的逐個(gè)遍歷的，其通過(guò)實(shí)現(xiàn)__next__函數(shù)得以逐個(gè)的訪問(wèn)關(guān)聯(lián)的數(shù)據(jù)元素;同時(shí)通過(guò)實(shí)現(xiàn)__iter__來(lái)實(shí)現(xiàn)對(duì)可迭代對(duì)象的兼容;

生成器是一種迭代器模式，其實(shí)現(xiàn)了數(shù)據(jù)的惰性生成，即只有使用的時(shí)候才會(huì)生成對(duì)應(yīng)的元素；

二、序列的可迭代性

python內(nèi)置的序列可以通過(guò)for進(jìn)行迭代，解釋器會(huì)調(diào)用iter函數(shù)獲取序列的迭代器，由于iter函數(shù)兼容序列實(shí)現(xiàn)的__getitem__，會(huì)自動(dòng)創(chuàng)建一個(gè)迭代器;

迭代器的

import re
from dis import dis
class WordAnalyzer:
    reg_word = re.compile('\w+')
    def __init__(self, text):
        self.words = self.__class__.reg_word.findall(text)
    def __getitem__(self, index):
        return self.words[index]
def iter_word_analyzer():
    wa = WordAnalyzer('this is mango word analyzer')
    print('start for wa')
    for w in wa:
        print(w)
    print('start while wa_iter')
    wa_iter = iter(wa)
    while True:
        try:
            print(next(wa_iter))
        except StopIteration as e:
            break;
iter_word_analyzer()
dis(iter_word_analyzer)
# start for wa
# this
# is
# mango
# word
# analyzer
# start while wa_iter
# this
# is
# mango
# word
# analyzer
#  15           0 LOAD_GLOBAL              0 (WordAnalyzer)
#               2 LOAD_CONST               1 ('this is mango word analyzer')
#               4 CALL_FUNCTION            1
#               6 STORE_FAST               0 (wa)
# 
#  16           8 LOAD_GLOBAL              1 (print)
#              10 LOAD_CONST               2 ('start for wa')
#              12 CALL_FUNCTION            1
#              14 POP_TOP
# 
#  17          16 LOAD_FAST                0 (wa)
#              18 GET_ITER
#         >>   20 FOR_ITER                12 (to 34)
#              22 STORE_FAST               1 (w)
# 
#  18          24 LOAD_GLOBAL              1 (print)
#              26 LOAD_FAST                1 (w)
#              28 CALL_FUNCTION            1
#              30 POP_TOP
#              32 JUMP_ABSOLUTE           20
# 
#  20     >>   34 LOAD_GLOBAL              1 (print)
#              36 LOAD_CONST               3 ('start while wa_iter')
#              38 CALL_FUNCTION            1
#              40 POP_TOP
# 
#  21          42 LOAD_GLOBAL              2 (iter)
#              44 LOAD_FAST                0 (wa)
#              46 CALL_FUNCTION            1
#              48 STORE_FAST               2 (wa_iter)
# 
#  23     >>   50 SETUP_FINALLY           16 (to 68)
# 
#  24          52 LOAD_GLOBAL              1 (print)
#              54 LOAD_GLOBAL              3 (next)
#              56 LOAD_FAST                2 (wa_iter)
#              58 CALL_FUNCTION            1
#              60 CALL_FUNCTION            1
#              62 POP_TOP
#              64 POP_BLOCK
#              66 JUMP_ABSOLUTE           50
# 
#  25     >>   68 DUP_TOP
#              70 LOAD_GLOBAL              4 (StopIteration)
#              72 JUMP_IF_NOT_EXC_MATCH   114
#              74 POP_TOP
#              76 STORE_FAST               3 (e)
#              78 POP_TOP
#              80 SETUP_FINALLY           24 (to 106)
# 
#  26          82 POP_BLOCK
#              84 POP_EXCEPT
#              86 LOAD_CONST               0 (None)
#              88 STORE_FAST               3 (e)
#              90 DELETE_FAST              3 (e)
#              92 JUMP_ABSOLUTE          118
#              94 POP_BLOCK
#              96 POP_EXCEPT
#              98 LOAD_CONST               0 (None)
#             100 STORE_FAST               3 (e)
#             102 DELETE_FAST              3 (e)
#             104 JUMP_ABSOLUTE           50
#         >>  106 LOAD_CONST               0 (None)
#             108 STORE_FAST               3 (e)
#             110 DELETE_FAST              3 (e)
#             112 RERAISE
#         >>  114 RERAISE
#             116 JUMP_ABSOLUTE           50
#         >>  118 LOAD_CONST               0 (None)
#             120 RETURN_VALUE

三、經(jīng)典的迭代器模式

標(biāo)準(zhǔn)的迭代器需要實(shí)現(xiàn)兩個(gè)接口方法，一個(gè)可以獲取下一個(gè)元素的__next__方法和直接返回self的__iter__方法；

迭代器迭代完所有的元素的時(shí)候會(huì)拋出StopIteration異常，但是python內(nèi)置的for、列表推到、元組拆包等會(huì)自動(dòng)處理這個(gè)異常；

實(shí)現(xiàn)__iter__主要為了方便使用迭代器，這樣就可以最大限度的方便使用迭代器；

迭代器只能迭代一次，如果需要再次迭代就需要再次調(diào)用iter方法獲取新的迭代器，這就要求每個(gè)迭代器維護(hù)自己的內(nèi)部狀態(tài)，即一個(gè)對(duì)象不能既是可迭代對(duì)象同時(shí)也是迭代器；

從經(jīng)典的面向?qū)ο笤O(shè)計(jì)模式來(lái)看，可迭代對(duì)象可以隨時(shí)生成自己關(guān)聯(lián)的迭代器，而迭代器負(fù)責(zé)具體的元素的迭代處理；

import re
from dis import dis
class WordAnalyzer:
    reg_word = re.compile('\w+')
    def __init__(self, text):
        self.words = self.__class__.reg_word.findall(text)
    def __iter__(self):
        return WordAnalyzerIterator(self.words)
class WordAnalyzerIterator:
    def __init__(self, words):
        self.words = words
        self.index = 0
    def __iter__(self):
        return self;
    def __next__(self):
        try:
            word = self.words[self.index]
        except IndexError:
            raise StopIteration()
        self.index +=1
        return word
def iter_word_analyzer():
    wa = WordAnalyzer('this is mango word analyzer')
    print('start for wa')
    for w in wa:
        print(w)
    print('start while wa_iter')
    wa_iter = iter(wa)
    while True:
        try:
            print(next(wa_iter))
        except StopIteration as e:
            break;
iter_word_analyzer()
# start for wa
# this
# is
# mango
# word
# analyzer
# start while wa_iter
# this
# is
# mango
# word
# analyzer

四、生成器也是迭代器

生成器是調(diào)用生成器函數(shù)生成的，生成器函數(shù)是含有yield的工廠函數(shù)；

生成器本身就是迭代器，其支持使用next函數(shù)遍歷生成器，同時(shí)遍歷完也會(huì)拋出StopIteration異常；

生成器執(zhí)行的時(shí)候會(huì)在yield語(yǔ)句的地方暫停，并返回yield右邊的表達(dá)式的值；

def gen_func():
    print('first yield')
    yield 'first'
    print('second yield')
    yield 'second'
print(gen_func)
g = gen_func()
print(g)
for val in g:
    print(val)
g = gen_func()
print(next(g))
print(next(g))
print(next(g))
# <function gen_func at 0x7f1198175040>
# <generator object gen_func at 0x7f1197fb6cf0>
# first yield
# first
# second yield
# second
# first yield
# first
# second yield
# second
# StopIteration

我們可以將__iter__作為生成器函數(shù)

import re
from dis import dis
class WordAnalyzer:
    reg_word = re.compile('\w+')
    def __init__(self, text):
        self.words = self.__class__.reg_word.findall(text)
    def __iter__(self):
        for word in self.words:
            yield word
def iter_word_analyzer():
    wa = WordAnalyzer('this is mango word analyzer')
    print('start for wa')
    for w in wa:
        print(w)
    print('start while wa_iter')
    wa_iter = iter(wa)
    while True:
        try:
            print(next(wa_iter))
        except StopIteration as e:
            break;
iter_word_analyzer()
# start for wa
# this
# is
# mango
# word
# analyzer
# start while wa_iter
# this
# is
# mango
# word
# analyzer

五、實(shí)現(xiàn)惰性迭代器

迭代器的一大亮點(diǎn)就是通過(guò)__next__來(lái)實(shí)現(xiàn)逐個(gè)元素的遍歷，這個(gè)大數(shù)據(jù)容器的遍歷帶來(lái)了可能性；

我們以前的實(shí)現(xiàn)在初始化的時(shí)候，直接調(diào)用re.findall得到了所有的序列元素，并不是一個(gè)很好的實(shí)現(xiàn)；我們可以通過(guò)re.finditer來(lái)在遍歷的時(shí)候得到數(shù)據(jù)；

import re
from dis import dis
class WordAnalyzer:
    reg_word = re.compile('\w+')
    def __init__(self, text):
        # self.words = self.__class__.reg_word.findall(text)
        self.text = text
    def __iter__(self):
        g = self.__class__.reg_word.finditer(self.text)
        print(g)
        for match in g:
            yield match.group()
def iter_word_analyzer():
    wa = WordAnalyzer('this is mango word analyzer')
    print('start for wa')
    for w in wa:
        print(w)
    print('start while wa_iter')
    wa_iter = iter(wa)
    wa_iter1= iter(wa)
    while True:
        try:
            print(next(wa_iter))
        except StopIteration as e:
            break;
iter_word_analyzer()
# start for wa
# <callable_iterator object at 0x7feed103e040>
# this
# is
# mango
# word
# analyzer
# start while wa_iter
# <callable_iterator object at 0x7feed103e040>
# this
# is
# mango
# word
# analyzer

六、使用生成器表達(dá)式簡(jiǎn)化惰性迭代器

生成器表達(dá)式是生成器的聲明性定義，與列表推到的語(yǔ)法類(lèi)似，只是生成元素是惰性的；

def gen_func():
    print('first yield')
    yield 'first'
    print('second yield')
    yield 'second'
l = [x for x in gen_func()]
for x in l:
    print(x)
print()
ge = (x for x in gen_func())
print(ge)
for x in ge:
    print(x)
# first yield
# second yield
# first
# second
#
# <generator object <genexpr> at 0x7f78ff5dfd60>
# first yield
# first
# second yield
# second

使用生成器表達(dá)式實(shí)現(xiàn)word analyzer

import re
from dis import dis
class WordAnalyzer:
    reg_word = re.compile('\w+')
    def __init__(self, text):
        # self.words = self.__class__.reg_word.findall(text)
        self.text = text
    def __iter__(self):
        # g = self.__class__.reg_word.finditer(self.text)
        # print(g)
        # for match in g:
        #     yield match.group()
        ge = (match.group() for match in self.__class__.reg_word.finditer(self.text))
        print(ge)
        return ge
def iter_word_analyzer():
    wa = WordAnalyzer('this is mango word analyzer')
    print('start for wa')
    for w in wa:
        print(w)
    print('start while wa_iter')
    wa_iter = iter(wa)
    while True:
        try:
            print(next(wa_iter))
        except StopIteration as e:
            break;
iter_word_analyzer()
# start for wa
# <generator object WordAnalyzer.__iter__.<locals>.<genexpr> at 0x7f4178189200>
# this
# is
# mango
# word
# analyzer
# start while wa_iter
# <generator object WordAnalyzer.__iter__.<locals>.<genexpr> at 0x7f4178189200>
# this
# is
# mango
# word
# analyzer