面向对象的学生图书馆2

Question

面向对象的学生图书馆随访

问:您如何重构这段代码，使其成为pythonic的，遵循OOP，阅读更好，并且是可管理的？如何更好地编写名称、函数和类？您如何知道需要使用哪种数据结构才能有效地管理数据？

from collections import defaultdict
from datetime import datetime, timedelta

class StudentDataBaseException(Exception): pass
class NoStudent(StudentDataBaseException): pass
class NoBook(StudentDataBaseException): pass

"""To keep of a record of students 
 who have yet to return books and their due dates"""
class CheckedOut:
     loan_period = 10
     fine_per_day = 2

     def __init__(self):
         self.due_dates = {}

     def check_in(self, name):
          due_date = datetime.now() + timedelta(days=self.loan_period)
          self.due_dates[name] = due_date

     def check_out(self, name):
         current_date = datetime.now()
         if current_date > self.due_dates[name]:
             delta = current_date - self.due_dates[name]
             overdue_fine = self.fine_per_day * delta.days
             print("Fine Amount: ", overdue_fine)

# This only contains the title name for now
class BookStatus:
    def __init__(self, title):
        self.title = title

    def __repr__(self):
        return self.title

    def __hash__(self):
        return 0

    def __eq__(self, other):
        return self.title == other

# contains a set of books
class Library:
    record = CheckedOut()

    def __init__(self):
        self.books = set()

    def add_book(self, new_book):
        self.books.add(new_book)

    def display_books(self):
        if self.books:
            print("The books we have made available in our library are:\n")
            for book in self.books:
                print(book)
        else:
            print("Sorry, we have no books available in the library at the moment")

    def lend_book(self, requested_book):
        if requested_book in self.books:
            print(f'''You have now borrowed \"{requested_book}\"''')
            self.books.remove(requested_book)
            return True

        else:
            print(f'''Sorry, \"{requested_book}\" is not there in our library at the moment''')
            return False

# container for students
class StudentDatabase:
    def __init__(self):
        self.books = defaultdict(set)

    def borrow_book(self, name, book, library):
        if library.lend_book(book):
            self.books[name].add(book)
            return True
        return False

    def return_book(self, name, book, library):
        if book not in self.books[name]:
            raise NoBook(f'''\"{name}\" doesn't seem to have borrowed "{book}"''')
            return False
        else:
            library.add_book(book)
            self.books[name].remove(book)
            return True

    def students_with_books(self):
        for name, books in self.books.items():
            if books:
                yield name, books


def borrow_book(library, book_tracking):
    name = input("Student Name: ")
    book = BookStatus(input("Book Title: "))

    if book_tracking.borrow_book(name, book, library):
        library.record.check_in(name)

def return_book(library, book_tracking):
    name = input("Student Name: ")
    returned_book = BookStatus(input("Book Title: "))

    if book_tracking.return_book(name, returned_book, library):
        library.record.check_out(name)


line = "_" * 100
menu = "Library Management System \n\n \
1) Add Book \n \
2) Display all Books \n \
3) Borrow a Book \n \
4) Return a Book \n \
5) Lending Record \n \
6) Exit"

library = Library()
book_tracking = StudentDatabase()

while True:
    print(line)
    print(menu)

    choice = get_valid_choice(min=1, max=6)
    print(line)

    if choice == 1:
        library.add_book(BookStatus(input("Book Title: ")))

    elif choice == 2:
        library.display_books()

    elif choice == 3:
        borrow_book(library, book_tracking)

    elif choice == 4:
        return_book(library, book_tracking)

    elif choice == 5:
        students = tuple(book_tracking.students_with_books())
        if students:
            for name, book in students:
                print(f"{name}: {book}")
        else:
            print("No students have borrowed books at the moment")

    elif choice == 6:
        break

Answer 1

我认为你的代码的一般结构很差。

1. 即使借出一本书，图书馆也拥有一本书。
2. 您的代码似乎无法处理同名的书籍。
3. 在不运行代码的情况下，您似乎可以通过取出一本书，然后返回另一本书来偷书。
4. BookStatus实现得非常糟糕。我会使用dataclasses。
5. 你实际上是在创建内存数据库。所以你应该设计数据库，然后围绕着它的代码。

从数据库设计开始，您有三件事：

• 书
• 贷款
• 人物

是的，没有图书馆，因为所有的书都在你的图书馆里。每本书都可以借很多次，但只能借一本书。每个人可以有多个贷款，但每个贷款只能给一个人。

因此，您的Loan对象应该具有图书和person的ID，但是图书和person都不应该有任何指向其他数据库项的链接。

然后，我们可以在Python中创建这些对象和表。我正在使用dataclasses (Python 3.6)和typing快速构建对象。由于我们已经将typing用于dataclass，所以我决定将程序的其余部分完全输入，允许静态分析器(如形象化 )检查代码中的错误。这允许使用以下基本代码：

from dataclasses import dataclass
from typing import Optional, Set, Mapping, TypeVar, Dict, Type, Iterator
from datetime import datetime, timedelta

T = TypeVar('T')


@dataclass
class Book:
    id: int
    name: str


@dataclass
class Person:
    id: int
    name: str


@dataclass
class Loan:
    id: int
    book: Book
    person: Person
    checkout: datetime
    due: datetime
    checkin: Optional[datetime]


class Table(Mapping[int, T]):
    _db: Dict[int, T]

    def __init__(self, type: Type[T]) -> None:
        self._db = {}
        self._type = type

    def __getitem__(self, key: int) -> T:
        return self._db[key]

    def __iter__(self) -> Iterator[T]:
        return iter(self._db)

    def __len__(self) -> int:
        return len(self._db)


books = Table(Book)
people = Table(Person)
loans = Table(Loan)

这样我们就可以轻松地添加其他功能：

class Table(Mapping[int, T]):
    # Other code
    def add(self, *args, **kwargs) -> None:
        key = len(self)
        self._db[key] = self._type(key, *args, **kwargs)

    def display(self) -> None:
        for value in self.values():
            print(value)


def borrow_book(person: int, book: int, loan_days: int) -> None:
    checkout = datetime.now()
    loans.add(
        books[book],
        people[person],
        checkout,
        checkout + timedelta(days=loan_days),
        None
    )


def return_book(loan: int) -> None:
    loans[loan].checkin = datetime.now()


def display_active_loans() -> None:
    has_active = False
    for loan in loans.values():
        if loan.checkin is not None:
            continue
        has_active = True
        print(f'{loan.id}: {loan.person.name} -> {loan.book.name}')
    if not has_active:
        print('No active loans')

使用起来相当容易，只需使用is：

books.add('Title')
books.display()
people.add('Student')
people.display()
borrow_book(0, 0, 10)
display_active_loans()
return_book(0)
display_active_loans()

from dataclasses import dataclass
from typing import Optional, Set, Mapping, TypeVar, Dict, Type, Iterator
from datetime import datetime, timedelta

T = TypeVar('T')


@dataclass
class Book:
    id: int
    name: str


@dataclass
class Person:
    id: int
    name: str


@dataclass
class Loan:
    id: int
    book: Book
    person: Person
    checkout: datetime
    due: datetime
    checkin: Optional[datetime]


class Table(Mapping[int, T]):
    _db: Dict[int, T]

    def __init__(self, type: Type[T]) -> None:
        self._db = {}
        self._type = type

    def __getitem__(self, key: int) -> T:
        return self._db[key]

    def __iter__(self) -> Iterator[T]:
        return iter(self._db)

    def __len__(self) -> int:
        return len(self._db)

    def add(self, *args, **kwargs) -> None:
        key = len(self)
        self._db[key] = self._type(key, *args, **kwargs)

    def display(self) -> None:
        for value in self.values():
            print(value)


books = Table(Book)
people = Table(Person)
loans = Table(Loan)


def borrow_book(person: int, book: int, loan_days: int) -> None:
    checkout = datetime.now()
    loans.add(
        books[book],
        people[person],
        checkout,
        checkout + timedelta(days=loan_days),
        None
    )


def return_book(loan: int) -> None:
    loans[loan].checkin = datetime.now()


def display_active_loans() -> None:
    has_active = False
    for loan in loans.values():
        if loan.checkin is not None:
            continue
        has_active = True
        print(f'{loan.id}: {loan.person.name} -> {loan.book.name}')
    if not has_active:
        print('No active loans')