Python 生成器

# 普通函数：一次性返回所有结果
def get_squares(n):
    result = []
    for i in range(n):
        result.append(i ** 2)
    return result

# 生成器函数：每次只返回一个值
def gen_squares(n):
    for i in range(n):
        yield i ** 2

# 对比
print(get_squares(5))      # [0, 1, 4, 9, 16]
print(list(gen_squares(5)))  # [0, 1, 4, 9, 16]

def simple_generator():
    """简单生成器"""
    print("开始执行")
    yield 1
    print("继续执行")
    yield 2
    print("再次继续")
    yield 3
    print("执行结束")

# 创建生成器对象
gen = simple_generator()

# 每次调用 next() 继续执行
print(next(gen))  # 开始执行 -> 1
print(next(gen))  # 继续执行 -> 2
print(next(gen))  # 再次继续 -> 3
# print(next(gen))  # 执行结束 -> StopIteration

def count_up_to(n):
    """计数生成器"""
    count = 1
    while count <= n:
        print(f"生成: {count}")
        yield count
        count += 1

# 创建生成器（不会立即执行）
counter = count_up_to(3)
print("生成器已创建，但还未执行")

# 按需获取值
print(f"获取: {next(counter)}")
print(f"获取: {next(counter)}")
print(f"获取: {next(counter)}")

def fibonacci(n):
    """斐波那契数列生成器"""
    a, b = 0, 1
    count = 0
    while count < n:
        yield a
        a, b = b, a + b
        count += 1

# 使用生成器
for num in fibonacci(10):
    print(num, end=" ")
# 0 1 1 2 3 5 8 13 21 34

def read_file_line_by_line(filename):
    """逐行读取文件"""
    with open(filename, 'r', encoding='utf-8') as f:
        for line in f:
            yield line.strip()

# 使用示例（假设文件存在）
# for line in read_file_line_by_line('data.txt'):
#     print(line)

def infinite_counter(start=0):
    """无限计数生成器"""
    while True:
        yield start
        start += 1

# 取前10个
counter = infinite_counter(100)
for _ in range(10):
    print(next(counter), end=" ")
# 100 101 102 103 104 105 106 107 108 109

# 列表推导式 - 一次性创建所有元素
squares_list = [x**2 for x in range(1000000)]  # 占用大量内存

# 生成器表达式 - 按需生成
squares_gen = (x**2 for x in range(1000000))   # 几乎不占用内存

# 对比内存使用
import sys
print(f"列表大小: {sys.getsizeof(squares_list)} 字节")
print(f"生成器大小: {sys.getsizeof(squares_gen)} 字节")

# 使用生成器表达式
squares_gen = (x**2 for x in range(10))
for square in squares_gen:
    print(square, end=" ")
# 0 1 4 9 16 25 36 49 64 81

# 链式使用生成器表达式
data = range(100)

# 步骤1: 筛选偶数
evens = (x for x in data if x % 2 == 0)

# 步骤2: 计算平方
squares = (x**2 for x in evens)

# 步骤3: 筛选大于100的
result = (x for x in squares if x > 100)

# 只在最后遍历时才真正计算
print(list(result)[:5])

def sub_generator():
    """子生成器"""
    yield 1
    yield 2
    yield 3

def main_generator():
    """主生成器"""
    yield "开始"
    yield from sub_generator()
    yield "结束"

# 使用
for item in main_generator():
    print(item)

def flatten_manual(nested_list):
    """手动展平嵌套列表"""
    for sublist in nested_list:
        for item in sublist:
            yield item

def flatten_yield_from(nested_list):
    """使用 yield from 展平"""
    for sublist in nested_list:
        yield from sublist

# 测试
nested = [[1, 2], [3, 4, 5], [6]]

print("手动展平:", list(flatten_manual(nested)))
print("yield from:", list(flatten_yield_from(nested)))
# 结果相同: [1, 2, 3, 4, 5, 6]

def stateful_generator():
    """展示状态保存"""
    print("状态1: 开始")
    x = 10
    yield x
    
    print("状态2: x 的值仍然是", x)
    x += 5
    yield x
    
    print("状态3: x 现在是", x)
    x *= 2
    yield x
    
    print("状态4: 最终 x =", x)

gen = stateful_generator()

print("=== 第一次调用 ===")
print(next(gen))

print("\n=== 第二次调用 ===")
print(next(gen))

print("\n=== 第三次调用 ===")
print(next(gen))

def interactive_generator():
    """交互式生成器"""
    print("生成器启动")
    
    # 第一次必须 send(None)
    received = yield "准备好接收数据"
    print(f"收到: {received}")
    
    received = yield f"已处理 {received}"
    print(f"收到: {received}")
    
    yield f"最终处理 {received}"

gen = interactive_generator()

# 启动生成器
result = next(gen)  # 或 gen.send(None)
print(f"生成器返回: {result}")

# 发送数据
result = gen.send("Hello")
print(f"生成器返回: {result}")

result = gen.send("World")
print(f"生成器返回: {result}")

def read_lines(filename):
    """读取文件行"""
    with open(filename, 'r') as f:
        for line in f:
            yield line.strip()

def filter_comments(lines):
    """过滤注释行"""
    for line in lines:
        if not line.startswith('#'):
            yield line

def parse_csv(lines):
    """解析 CSV 格式"""
    for line in lines:
        yield line.split(',')

# 构建处理管道（假设文件存在）
# pipeline = parse_csv(filter_comments(read_lines('data.csv')))
# for row in pipeline:
#     print(row)

def paginated_data(page_size=10):
    """模拟分页加载数据"""
    total_records = 100
    current_page = 0
    
    while current_page * page_size < total_records:
        # 模拟从数据库加载
        start = current_page * page_size
        end = min(start + page_size, total_records)
        page_data = list(range(start, end))
        
        print(f"加载第 {current_page + 1} 页")
        yield page_data
        
        current_page += 1

# 使用
for page in paginated_data(20):
    print(f"处理: {page}")

def my_range(start, stop=None, step=1):
    """自定义 range 生成器"""
    if stop is None:
        stop = start
        start = 0
    
    if step == 0:
        raise ValueError("step 不能为 0")
    
    if step > 0:
        while start < stop:
            yield start
            start += step
    else:
        while start > stop:
            yield start
            start += step

# 测试
print("range(5):", list(my_range(5)))
print("range(1, 6):", list(my_range(1, 6)))
print("range(0, 10, 2):", list(my_range(0, 10, 2)))
print("range(10, 0, -2):", list(my_range(10, 0, -2)))