Python 入门教程（十）：面向对象编程

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class Book:
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    def __init__(self, title, price):
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        self.title = title
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        self.price = price
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    def __str__(self):                    # print / str() 时调用
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        return f"《{self.title}》¥{self.price}"
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    def __repr__(self):                   # 调试/容器内显示时调用
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        return f"Book({self.title!r}, {self.price})"
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    def __eq__(self, other):              # == 比较
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        return isinstance(other, Book) and self.title == other.title
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    def __lt__(self, other):              # < 比较（配合 sort）
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        return self.price < other.price
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    def __len__(self):                    # len() 行为
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        return len(self.title)
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    def __add__(self, other):             # + 行为
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        return self.price + other.price
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b1 = Book("Python入门", 59)
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b2 = Book("Python入门", 79)
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print(b1)                    # 《Python入门》¥59
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print(b1 == b2)              # True（同名即相等）
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books = [Book("A书", 49), Book("B书", 29)]
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books.sort()                 # 按 __lt__ 排序
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print(len(b1))               # 9（标题字符数）
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print(b1 + b2)               # 138（价格相加）

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from dataclasses import dataclass, field
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from datetime import date, timedelta
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class LibraryError(Exception):
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    """图书馆业务异常"""
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    pass
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@dataclass
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class Book:
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    isbn: str
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    title: str
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    copies: int = 1
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    borrowers: list = field(default_factory=list)
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    @property
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    def available(self) -> int:
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        return self.copies - len(self.borrowers)
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class Member:
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    def __init__(self, mid: int, name: str):
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        self.mid = mid
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        self.name = name
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        self.borrowed: dict[str, date] = {}     # isbn -> 应还日期
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    def __str__(self):
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        return f"会员{self.mid}-{self.name}（在借{len(self.borrowed)}本）"
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class Library:
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    LOAN_DAYS = 30
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    MAX_BOOKS = 3
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    def __init__(self):
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        self._books: dict[str, Book] = {}
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        self._members: dict[int, Member] = {}
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    def add_book(self, isbn, title, copies=1):
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        if isbn in self._books:
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            self._books[isbn].copies += copies
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        else:
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            self._books[isbn] = Book(isbn, title, copies)
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    def register(self, name) -> Member:
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        mid = max(self._members, default=0) + 1
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        m = Member(mid, name)
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        self._members[mid] = m
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        return m
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    def borrow(self, member: Member, isbn: str):
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        """借书：封装所有业务规则校验"""
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        book = self._books.get(isbn)
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        if book is None:
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            raise LibraryError(f"无此书: {isbn}")
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        if book.available <= 0:
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            raise LibraryError(f"《{book.title}》已全部借出")
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        if len(member.borrowed) >= self.MAX_BOOKS:
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            raise LibraryError(f"{member.name} 已达借阅上限{self.MAX_BOOKS}本")
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        book.borrowers.append(member.name)
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        member.borrowed[isbn] = date.today() + timedelta(days=self.LOAN_DAYS)
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    def return_book(self, member: Member, isbn: str):
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        if isbn not in member.borrowed:
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            raise LibraryError(f"{member.name} 没借这本书")
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        self._books[isbn].borrowers.remove(member.name)
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        del member.borrowed[isbn]
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    def __len__(self):                          # len(图书馆) = 藏书种数
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        return len(self._books)
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    def __contains__(self, isbn):               # isbn in 图书馆
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        return isbn in self._books
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# —— 使用 ——
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lib = Library()
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lib.add_book("978-7-111-40701-0", "Python编程：从入门到实践", copies=2)
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lib.add_book("978-7-115-53608-2", "算法（第4版）")
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ming = lib.register("小明")
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lib.borrow(ming, "978-7-111-40701-0")
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print(ming)                                   # 会员1-小明（在借1本）
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print(f"馆藏 {len(lib)} 种；'978-7-115-53608-2' 在馆: {'978-7-115-53608-2' in lib}")
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lib.return_book(ming, "978-7-111-40701-0")
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print(ming)                                   # 会员1-小明（在借0本）