一文搞懂python中的迭代器和生成器

# 这个“列表”为斐波那契数列，表示该数列的迭代器如下：
class Fibs:
    def __init__(self):
        self.a = 0
        self.b = 1
    def __next__(self):
        self.a, self.b = self.b, self.a + self.b
        return self.a  # 前面逻辑自定义,最后返回下一个值即可
    def __iter__(self):
        return self  # 返回迭代器本身(一个包含 __next__ 方法的对象)
    
fibs = Fibs()
for f in fibs:
	if f > 1000:
		print(f)  # 1597
		break  # 若不中断循环,将一直循环下去
        
next(fibs)  # 2584
next(fibs)  # 4181

it = iter([1, 2, 3])  # list 是可迭代对象哦
next(it)  # 1
next(it)  # 2
next(it)  # 3
next(it)  # StopIretation; 普通的可迭代对象是可复用的,而迭代器是一次性的,回不了头的

it = iter("ABCD")  # string 也是可迭代对象
for i in it:
    print(i, end=" ")  # A B C D
for i in it:
    print(i, end=" ")  # ⽆输出

# ⽅法1：dir()查看__iter__,详情请自己尝试
dir(2) # 没有
dir("abc") # 有 __iter__()
# ⽅法2：isinstance()判断
import collections
isinstance(2, collections.Iterable) # False
isinstance("abc", collections.Iterable) # True
# ⽅法3：hasattr()判断
hasattr(2,"__iter__") # False
hasattr("abc","__iter__") # True
# ⽅法4：⽤iter()查看是否报错
iter(2) # 报错：'int' object is not iterable
iter("abc") # <str_iterator at 0x1e2396d8f28>

# 使用构造函数 list() 显示的将迭代器转换为列表
class TestIterator:
    value = 0
    
    def __next__(self):
        self.value += 1
        if self.value > 10: raise StopIteration
        return self.value
    def __iter__(self):
        return self
ti = TestIterator()
ti2 = list(ti)  # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for i in ti2:
    print(i, end=" ")  # 1 2 3 4 5 6 7 8 9 10
        
print('the second:')

# 斐波纳契数列的生成器实现: 返回数列的前 n 项
def fibs(n):
    a, b = 0, 1
    for _ in range(n):
        yield a  # 返回的是一个生成器
        a, b = b, b+a
f = fibs(5)
print(f)  # <generator object fibs at 0x05BB20B0>
print(list(f))  # [0, 1, 1, 2, 3]; 此处生成器 f 已经被迭代过一次了
for i in f:
    print(i, end=" ")  # ⽆输出; for循环会⾃动捕捉到 StopIteration 异常并停⽌调⽤ next()
    
print(next(f))  # StopIteration

L = [x*x for x in range(10)]  # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
g = (x*x for x in range(10))  # <generator object <genexpr> at 0x052AF8F0>
print(next(g))  # 0
sum(i ** 2 for i in range(10))  # 285

nested = [[1, 2], [3, 4], [5], 6]
def flatten(nested):
    try:
        for sub in nested:
            for ele in sub:
                yield ele
    except TypeError:
        yield sub
            
f = flatten(nested)
next(f)  # 1
# print(list(f))  # [2, 3, 4, 5, 6]
for i in f3:
    print(i)  # 2 3 4 5 6

def flatten(nested):
    try:
        for sub in nested:
            for ele in flatten(sub):
                yield ele
    except TypeError:
        yield nested
        
nested = [[[1], 2], 3, 4, [5, [6, 7]], 8]
print(list(flatten(nested)))

s = 'ABCD'
s2 = s[0]  # 'A'
s2[0]  # 'A'

def flatten(nested):    
    try:
        if isinstance(nested, str): raise TypeError
            
        for sub in nested:
            for ele in flatten(sub):
                yield ele
    except TypeError:
        yield nested
        
nested = [[[1], '23'], 3, '43', [5, [6, '73']], 8]
print(list(flatten(nested)))  # [1, '23', 3, '43', 5, 6, '73', 8]

def flatten(nested):
	try:
		for sublist in nested:
			for element in flatten(sublist):
				print("element:", element)
				yield element
	except TypeError:
		print("nested :", nested)
		yield nested
		
print(list(flatten([[1, 2], [3, 4], [5], 6])))