从列表到生成器详解Python可迭代对象的完全指南

# 列表是可迭代对象
for item in [1, 2, 3]:
    print(item)  # 依次输出 1, 2, 3

# 字符串是可迭代对象
for char in "hello":
    print(char)  # 依次输出 h, e, l, l, o

# 字典是可迭代对象
for key in {"name": "Tom", "age": 18}:
    print(key)  # 依次输出 name, age

# 查看列表的__iter__方法
print([].__iter__)  # <method-wrapper '__iter__' of list object at 0x...>

# 列表
for i in [1, 2, 3]:
    print(i)

# 元组
for i in (1, 2, 3):
    print(i)

# 字符串
for c in "abc":
    print(c)

# range
for i in range(3):
    print(i)  # 0, 1, 2

# 集合（遍历的是键）
for i in {1, 2, 3}:
    print(i)

# 字典（默认遍历键）
for key in {"name": "Tom", "age": 18}:
    print(key)  # name, age

it = iter([1, 2, 3])
for i in it:
    print(i)  # 1, 2, 3

# 二次使用：什么都不输出，也不会报错
for i in it:
    print(i)  # 不输出

# 列表 → 直接存值
lst = [1, 2, 3]

# 生成器 → 记录生成规则，用next()才取值
gen = (x * 2 for x in range(3))
print(next(gen))  # 0
print(next(gen))  # 2
print(next(gen))  # 4

# 生成器表达式
gen = (i * 2 for i in range(3))
for i in gen:
    print(i)  # 0, 2, 4

# range 对象
for i in range(5):
    print(i)

from collections.abc import Iterable

# 测试常见对象
print(isinstance([1, 2, 3], Iterable))  # True - 列表
print(isinstance("hello", Iterable))     # True - 字符串
print(isinstance((1, 2, 3), Iterable))   # True - 元组
print(isinstance({1, 2, 3}, Iterable))   # True - 集合
print(isinstance({"a": 1}, Iterable))   # True - 字典
print(isinstance(range(5), Iterable))    # True - range
print(isinstance((x for x in range(3)), Iterable))  # True - 生成器

# 非可迭代对象
print(isinstance(100, Iterable))          # False - 数字
print(isinstance(None, Iterable))         # False - None

# 列表有 __iter__ 方法
print(hasattr([1, 2, 3], '__iter__'))  # True

# 数字没有
print(hasattr(100, '__iter__'))  # False

graph LR
    A[可迭代对象<br/>Iterable] -->|实现__iter__| B[返回迭代器]
    B --> C[迭代器<br/>Iterator]
    C -->|实现__next__| D[逐个取值]
    
    style A fill:#e3f2fd,stroke:#1976d2
    style C fill:#ffebee,stroke:#d32f2f

# 列表 → 可迭代对象
lst = [1, 2, 3]
print(hasattr(lst, '__iter__'))  # True
print(hasattr(lst, '__next__'))  # False

# iter() 返回的是迭代器
it = iter(lst)
print(hasattr(it, '__iter__'))  # True
print(hasattr(it, '__next__'))  # True

# 用 next() 获取值
print(next(it))  # 1
print(next(it))  # 2
print(next(it))  # 3
# print(next(it))  # StopIteration 异常

# iter(可迭代对象) → 返回迭代器
lst = [1, 2, 3]
it = iter(lst)  # 获取迭代器
print(next(it))  # 1

# 也可以直接用 iter(obj)
# 相当于调用 obj.__iter__()

# 方式一：用 iter() 返回迭代器
class MyIterable:
    def __init__(self, data):
        self.data = data
    
    def __iter__(self):
        # 返回迭代器对象
        return iter(self.data)

# 方式二：同时实现 __iter__ + __next__，返回 self
class MyIterable2:
    def __init__(self, data):
        self.data = data
        self.index = 0
    
    def __iter__(self):
        return self  # 返回自身
    
    def __next__(self):
        if self.index >= len(self.data):
            raise StopIteration
        value = self.data[self.index]
        self.index += 1
        return value

# 使用
obj = MyIterable([1, 2, 3])
for i in obj:
    print(i)  # 1, 2, 3

class MyIterable3:
    def __init__(self, data):
        self.data = data
    
    def __getitem__(self, index):
        return self.data[index]

# Python 会自动从 index 0 开始尝试调用 __getitem__
obj = MyIterable3([1, 2, 3])
for i in obj:
    print(i)  # 1, 2, 3

class MyIterator:
    def __init__(self, data):
        self.data = data
        self.index = 0
    
    def __iter__(self):
        return self  # 返回自身
    
    def __next__(self):
        if self.index >= len(self.data):
            raise StopIteration  # 迭代结束
        value = self.data[self.index]
        self.index += 1
        return value

# 使用
it = MyIterator([1, 2, 3])
print(next(it))  # 1
print(next(it))  # 2
print(next(it))  # 3
# next(it)  # StopIteration

# 用 yield 的函数 → 生成器
def my_generator():
    yield 1
    yield 2
    yield 3

# 调用函数不会执行，只是返回生成器对象
gen = my_generator()
print(gen)  # <generator object my_generator at ...>

# 生成器本身就是迭代器，不需要 iter()
for i in gen:
    print(i)  # 1, 2, 3