46. Python条件语句和循环语句

# Python中的真假值判定规则
# 以下值被判定为False:
# False、None、数值0(0, 0.0)、空字符串('')、空容器([], {}, set())

# 示例:检查持仓是否为空
position = 0
if position:
    print(f'持仓{position}股')
else:
    print('空仓')  # 会执行这个分支

# 显式比较(推荐写法,更清晰)
if position != 0:
    print(f'持仓{position}股')
else:
    print('空仓')

# 1. 链式比较(Python风格,更简洁)
price = 15.0

if 10 < price < 20:
    print('价格在10到20之间')

# 等价于: if price > 10 and price < 20

# 2. 三元运算符: 值1 if 条件 else 值2
amount = 10000
commission_rate = 0.0003 if amount > 10000 else 0.0005
print(f'手续费率: {commission_rate:.4f}')

# 等价的if-else语句:
# if amount > 10000:
#     commission_rate = 0.0003
# else:
#     commission_rate = 0.0005

# 不推荐:深层嵌套,可读性差
price = 18.5
volume = 1000000
ma20 = 18.00

# 推荐:提前返回(卫语句),逻辑更清晰
def generate_signal(price, volume, ma20):
    if price <= 18:
        return '不操作'
    if volume <= 500000:
        return '观望'
    if price > ma20:
        return '强力买入'
    return '考虑买入'

signal = generate_signal(price, volume, ma20)
print(f'交易信号: {signal}')

# 股票筛选策略
stock = {
    'name': '招商银行',
    'price': 45.2,
    'pe': 8.5,      # 市盈率
    'pb': 0.8,      # 市净率
    'roe': 0.15     # 净资产收益率
}

# 多条件筛选
conditions = [
    stock['price'] < 100,
    stock['pe'] < 10,
    stock['pb'] < 1,
    stock['roe'] > 0.10
]

# all()函数:所有条件为True时返回True
if all(conditions):
    print(f'{stock["name"]} 符合价值投资标准')
else:
    print(f'{stock["name"]} 不符合标准')

# range()生成整数序列,常用于for循环
# 语法: range(start, stop, step)

# 用法1:指定结束值(从0开始)
print('range(5):', list(range(5)))

# 用法2:指定起始和结束值
print('range(2, 5):', list(range(2, 5)))

# 用法3:指定步长
print('range(0, 10, 2):', list(range(0, 10, 2)))

# 注意:range()不包含结束值(左闭右开)

# zip()可以同时遍历多个序列
codes = ['600519.SH', '000858.SZ', '600036.SH']
prices = [1850, 220, 45]

# 同时遍历代码和价格
for code, price in zip(codes, prices):
    print(f'{code}: {price}元')

print()

# 使用enumerate和zip组合
for i, (code, price) in enumerate(zip(codes, prices)):
    print(f'{i+1}. {code}: {price}元')

portfolio = {
    '600519.SH': {'name': '茅台', 'shares': 100},
    '000858.SZ': {'name': '五粮液', 'shares': 200}
}

# 方式1:遍历键
print('股票代码:')
for code in portfolio:
    print(f'  {code}')

# 方式2:遍历键值对
print('\n股票详情:')
for code, info in portfolio.items():
    print(f'  {code}: {info["name"]}, {info["shares"]}股')

# 方式3:遍历值
print('\n股票名称:')
for info in portfolio.values():
    print(f'  {info["name"]}')

# 列表推导式是for循环的简洁表达
# 语法: [expression for item in iterable if condition]

# 传统for循环
squares = []
for i in range(10):
    squares.append(i ** 2)
print('传统循环:', squares[:5], '...')

# 列表推导式(Python风格,更简洁)
squares = [i ** 2 for i in range(10)]
print('列表推导:', squares[:5], '...')

# 带条件的列表推导式
even_squares = [i ** 2 for i in range(10) if i % 2 == 0]
print('偶数平方:', even_squares)

# 金融应用:筛选符合条件的股票
stocks = [
    {'name': '茅台', 'pe': 45},
    {'name': '五粮液', 'pe': 35},
    {'name': '招行', 'pe': 8}
]

# 找出PE<20的股票
value_stocks = [s for s in stocks if s['pe'] < 20]
print(f'价值股: {[s["name"] for s in value_stocks]}')

# 提取所有股票名称
names = [s['name'] for s in stocks]
print(f'所有股票: {names}')

# 模式1:计数器循环
count = 0
while count < 3:
    print(f'计数: {count}')
    count += 1

# 模式2:while-else结构
# else在循环正常结束时执行(break跳出则不执行)
count = 0
while count < 3:
    count += 1
else:
    print(f'循环正常结束, count={count}')

import random

# 模拟股价路径,直到达到止损或止盈
random.seed(42)  # 设置随机种子,保证结果可复现
price = 100
stop_loss = 95     # 止损价
take_profit = 105  # 止盈价
days = 0

while stop_loss < price < take_profit:
    # 每日收益率服从正态分布(均值0.001,标准差0.02)
    daily_return = random.gauss(0.001, 0.02)
    price *= (1 + daily_return)
    days += 1

    if days > 252:  # 最多模拟一年(252个交易日)
        break

if price >= take_profit:
    print(f'第{days}天止盈, 价格: {price:.2f}元')
elif price <= stop_loss:
    print(f'第{days}天止损, 价格: {price:.2f}元')
else:
    print(f'第{days}天未达目标, 价格: {price:.2f}元')

# break只跳出最近的一层循环
for i in range(3):
    for j in range(3):
        if j == 1:
            break  # 只跳出内层循环
        print(f'i={i}, j={j}')
# 每次内层循环在j=1时中断
# 外层循环继续执行

# pass语句什么都不做,用作占位符

# 场景1:空函数体(待实现)
def placeholder_function():
    pass  # TODO: 待实现

# 场景2:条件分支占位
price = 50
if price > 100:
    pass  # 待添加处理逻辑
else:
    print(f'价格{price}元,低于100元')

# pass在开发阶段非常有用,保持代码结构完整

stocks = [
    {'code': '600519.SH', 'price': 1850, 'pe': 45},
    {'code': '000858.SZ', 'price': 220, 'pe': 35},
    {'code': '600036.SH', 'price': 45, 'pe': 8}
]

# 方法1:单个复合条件
print('方法1 - 复合条件:')
for stock in stocks:
    if stock['price'] < 500 and stock['pe'] < 40:
        print(f"  {stock['code']}: PE={stock['pe']}")

# 方法2:列表推导式(最Pythonic)
print('\n方法2 - 列表推导式:')
filtered = [s for s in stocks if s['price'] < 500 and s['pe'] < 40]
for stock in filtered:
    print(f"  {stock['code']}: PE={stock['pe']}")

stocks = [
    {'code': '600519.SH', 'price': 1850, 'pe': 45, 'pb': 10, 'dividend': 1.5},
    {'code': '000858.SZ', 'price': 220, 'pe': 35, 'pb': 8, 'dividend': 1.2},
    {'code': '600036.SH', 'price': 45, 'pe': 8, 'pb': 0.8, 'dividend': 2.5},
    {'code': '601318.SH', 'price': 50, 'pe': 9, 'pb': 0.9, 'dividend': 2.0}
]

# 策略1:价值投资(低PE低PB)
print('=== 价值投资策略 ===')
for stock in stocks:
    if stock['pe'] < 20 and stock['pb'] < 1:
        print(f"  {stock['code']}: PE={stock['pe']}, PB={stock['pb']}")

# 策略2:红利策略(高股息)
print('\n=== 红利策略 ===')
for stock in stocks:
    if stock['dividend'] > 2.0:
        print(f"  {stock['code']}: 股息率={stock['dividend']}%")

positions = [
    {'code': '600519.SH', 'entry_price': 1800, 'current_price': 1850},
    {'code': '000858.SZ', 'entry_price': 230, 'current_price': 220},
    {'code': '600036.SH', 'entry_price': 48, 'current_price': 45}
]

print('止损信号:')
for pos in positions:
    # 计算收益率
    ret = (pos['current_price'] - pos['entry_price']) / pos['entry_price']
    # 计算亏损金额
    loss = pos['entry_price'] - pos['current_price']

    # 检查止损条件
    if ret < -0.05 and loss > 3:
        print(f"  {pos['code']}: 亏损{ret:.2%}, 亏损{loss:.0f}元, 建议止损")
    elif ret < 0:
        print(f"  {pos['code']}: 亏损{ret:.2%}, 继续持有")
    else:
        print(f"  {pos['code']}: 盈利{ret:.2%}, 继续持有")