Python match/case 结构化模式匹配：从入门到真正会用

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from dataclasses import dataclass
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@dataclass
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class Point:
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    x: float
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    y: float
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@dataclass
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class Circle:
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    center: Point
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    radius: float
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@dataclass
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class Rectangle:
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    top_left: Point
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    bottom_right: Point
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def describe_shape(shape):
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    match shape:
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        case Circle(center=Point(x=0, y=0), radius=r):
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            print(f"原点处的圆，半径 {r}")
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        case Circle(center=Point(x=x, y=y), radius=r) if r > 100:
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            print(f"大圆：圆心 ({x},{y})，半径 {r}")
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        case Circle(center=center, radius=r):
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            print(f"圆：圆心 {center}，半径 {r}")
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        case Rectangle(top_left=Point(x=x1, y=y1), bottom_right=Point(x=x2, y=y2)):
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            area = abs(x2 - x1) * abs(y2 - y1)
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            print(f"矩形面积: {area}")

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import sys
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def main():
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    match sys.argv[1:]:
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        case ["serve", "--port", port]:
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            start_server(int(port))
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        case ["serve"]:
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            start_server(8000)
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        case ["build", "--output", path]:
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            build_project(path)
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        case ["build"]:
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            build_project("./dist")
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        case ["deploy", env] if env in ("staging", "production"):
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            deploy(env)
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        case ["deploy", env]:
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            print(f"未知环境: {env}，只支持 staging/production")
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        case ["--help" | "-h"]:
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            print_help()
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        case []:
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            print_help()
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        case _:
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            print(f"未知命令: {' '.join(sys.argv[1:])}")

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def evaluate(expr):
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    match expr:
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        case int(n) | float(n):
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            return n
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        case ("+", left, right):
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            return evaluate(left) + evaluate(right)
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        case ("-", left, right):
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            return evaluate(left) - evaluate(right)
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        case ("*", left, right):
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            return evaluate(left) * evaluate(right)
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        case ("/", left, right):
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            divisor = evaluate(right)
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            if divisor == 0:
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                raise ValueError("除零错误")
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            return evaluate(left) / divisor
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        case ("neg", operand):
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            return -evaluate(operand)
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        case _:
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            raise ValueError(f"无法解析的表达式: {expr}")
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# 使用：(+ (* 3 4) 5) = 17
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result = evaluate(("+", ("*", 3, 4), 5))
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print(result)  # 17

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def process_order(order, event):
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    match (order["status"], event):
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        case ("pending", {"type": "pay", "amount": amount}):
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            order["status"] = "paid"
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            order["paid_amount"] = amount
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            print(f"订单已支付: ¥{amount}")
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        case ("paid", {"type": "ship", "tracking": tracking}):
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            order["status"] = "shipped"
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            order["tracking"] = tracking
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            print(f"已发货，运单号: {tracking}")
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        case ("shipped", {"type": "deliver"}):
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            order["status"] = "delivered"
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            print("已签收")
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        case ("delivered", {"type": "refund", "reason": reason}):
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            order["status"] = "refunding"
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            print(f"申请退款: {reason}")
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        case (status, {"type": event_type}):
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            print(f"非法状态转换: {status} + {event_type}")

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# ❌ 错误：expected 不是比较，是赋值！
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expected = 200
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match status:
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    case expected:  # 这会匹配任何值，并把 status 赋给 expected
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        print("OK")
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# ✅ 正确方案一：用点号引用
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class Status:
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    OK = 200
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match status:
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    case Status.OK:
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        print("OK")
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# ✅ 正确方案二：用守卫条件
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match status:
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    case code if code == expected:
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        print("OK")
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# ✅ 正确方案三：用字面量
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match status:
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    case 200:
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        print("OK")