Python中迭代器和產生器的樣本詳解

迭代器

Iterable

定義

class Iterable(metaclass=ABCMeta):    __slots__ = ()    @abstractmethod    def __iter__(self):        while False:            yield None    @classmethod    def __subclasshook__(cls, C):        if cls is Iterable:            if any("__iter__" in B.__dict__ for B in C.__mro__):                return True        return NotImplemented

由定義可知Iterable必然包含__iter__函數

Iterator

定義

class Iterator(Iterable):    __slots__ = ()    @abstractmethod    def __next__(self):        'Return the next item from the iterator. When exhausted, raise StopIteration'        raise StopIteration    def __iter__(self):        return self    @classmethod    def __subclasshook__(cls, C):        if cls is Iterator:            if (any("__next__" in B.__dict__ for B in C.__mro__) and                any("__iter__" in B.__dict__ for B in C.__mro__)):                return True        return NotImplemented

從定義可知Iterator包含__next__和__iter__函數,當next超出範圍時將拋出StopIteration事件

類型關係

#! /usr/bin/python#-*-coding:utf-8-*-from collections import Iterator,Iterable# 迭代器s = 'abc'l = [1,2,3]d=iter(l)print(isinstance(s,Iterable)) # Trueprint(isinstance(l,Iterable)) # Trueprint(isinstance(s,Iterator)) # Falseprint(isinstance(l,Iterator)) # Falseprint(isinstance(d,Iterable)) # Trueprint(isinstance(d,Iterator)) # True

理論上你可以使用next()來執行__next__()，直到迭代器拋出StopIteration 實際上系統提供了for .. in ..的方式來解析迭代器

l = [1,2,3,4]for i in l:    print(i)    # 執行結果    # 1# 2# 3# 4

產生器 generator

產生器的本質是一個迭代器

#! /usr/bin/python#-*-coding:utf-8-*-from collections import Iterator,Iterables = (x*2 for x in range(5))print(s)print('Is Iterable:' + str(isinstance(s,Iterable)))print('Is Iterator:' + str(isinstance(s,Iterator)))for x in s:    print(x)# 執行結果    # <generator object <genexpr> at 0x000001E61C11F048># Is Iterable:True# Is Iterator:True# 0# 2# 4# 6# 8

函數中如果存在yield 則該函數是一個產生器對象 在每一次執行next函數時該函數會在上一個yield處開始執行，並在下一個yield處返回（相當於return）

def foo():    print("First")    yield 1    print("Second")    yield 2f = foo()print(f)a = next(f)print(a)b = next(f)print(b)# <generator object foo at 0x0000020B697F50F8># First# 1# Second# 2

執行個體

#! /usr/bin/python#-*-coding:utf-8-*-def add(s,x):    return s+xdef gen():    for i in range(4):        yield ibase = gen()# 由於gen函數中存在yield，所以# for 迴圈本質是建立了兩個generator object，而非執行函數# base = (add(i,10) for i in base)# base = (add(i,10) for i in base)for n in [1,10]:    base = (add(i,n) for i in base)# 這裡才開始展開產生器# 第一個產生器展開# base = (add(i,10) for i in base)# base = (add(i,10) for i in range(4))# base = (10,11,12,13)## 第二個產生器展開# base = (add(i,10) for i in (10,11,12,13))# base = (20,21,22,23)print(list(base)) # [20,21,22,23]