如何让我的 __str__() 函数的返回值在我的 main 中打印？

Question

我需要为一个程序创建两个类，该程序接受书籍、标题、作者、页数的输入变量，并查看它是否已检出，然后按以下格式将书籍添加到字典中 dictionary[title] = author .然后我需要以这种格式在主函数中打印我的所有信息：“”title author pages checkOut。”我基本上需要在我的类中打印出我的__str__()函数的返回值。这是我的代码：

class Book:
    def __init__(self, title, author, pages):
        self.title = title
        self.author = author
        self.pages = pages
        self.checkedOut = False

    def checked_Out(self):
        print(self.checkedOut)
        return self.checkedOut

    def change_value_of_checkedOut(self):
        if self.checkedOut == False:
            self.checkedOut = True
            print("Switched from False to True.")
        elif self.checkedOut == True:
            self.checkedOut = False
            print("Switched from True to False.")

    def return_pages(self):
        print(self.pages)
        return self.pages

    def return_title(self):
        print(self.title)
        return self.title

    def __str__(self):
        return ("Title: " + self.title + "Author: " + self.author +
                "Pages: " + self.pages + "Checked Out Status: " +
                self.checkedOut)

class Library:
    def __init__(self):
        self.collection = {}

    def addExistingBook(self, book):
        self.collection[book.title] = book.author

    def addNewBook(self, title, author, pages):
        new_book = Book(title, author, pages)
        self.collection[title] = new_book.author

    def change_checked_out_status(self, title):
        if title in self.collection:
            title.change_value_of_checkedOut()
        else:
            print("This book is not in the collection.")

    def __str__(self):
        for myBook in self.collection[myBook]:
            self.collection[myBook]
            self.collection[myBook] = self.author

            # I want to print this return value in my main
        return ("Title: " + self.title + "Author: " + self.author
                + "Pages: " + self.pages + "Checked Out Status: " + self.checkedOut)

def main():
    title = str(input("Enter the title of the book. "))
    author = str(input("Enter the author of the book. "))
    pages = int(input("Enter the number of pages in the book. "))
    myBook = Book(title, author, pages)
    myLib = Library()
    myLib.addExistingBook(myBook)
    myLib2 = Library()
    myLib3 = myLib2.__str__()
    print(myLib3)

main()

在类库中我的__str__(self) 函数中的 for 循环似乎有问题（我希望循环遍历集合中的任何一本书）但我不确定是什么问题是。这是我收到的错误消息：

Enter the title of the book. A Tale of Two Cities
Enter the author of the book. Charles Dickens
Enter the number of pages in the book. 434
Traceback (most recent call last):
  File "C:\Python33\Class Programs\lab8.py", line 71, in <module>
    main()
  File "C:\Python33\Class Programs\lab8.py", line 68, in main
    myLib3 = myLib2.__str__()
  File "C:\Python33\Class Programs\lab8.py", line 54, in __str__
    for myBook in self.collection[myBook]:
UnboundLocalError: local variable 'myBook' referenced before assignment

Answer 1

您正在遍历字典的键，您不应该尝试在循环语句中传递键：

def __str__(self):
    for myBook in self.collection:

目前尚不清楚您要对该循环做什么。然后您访问self.collection[myBook] 而不对返回值做任何事情，然后您将替换 值self.author（self 上不存在，这仍然是库），然后你返回与__str__() 结果完全不同的东西。

我认为您想在此处创建图书馆图书列表：

def __str__(self):
    return ', '.join('{} - {}'.format(title, author) for title, author in self.collection.items())

现在您返回一个以逗号分隔的列表，其中包含您收藏中的所有标题和作者。

那里发生了 3 件事：

• 使用生成器表达式，我们遍历字典.items() method；这将为字典中的所有内容生成 (key, value) 对。
• 每个键和值都传递给str.format() method，使用字符串作为模板，在您的self.collection 字典中存储的标题和作者对之间放置- 破折号。
• 整个循环传递给str.join() method；此方法从给定的序列中获取所有字符串，并将它们与它们之间的', ' 逗号连接在一起。

接下来，您正在创建一个 empty 库，然后打印__str__() 方法的输出；无需显式调用，print() 会为您执行此操作。不要创建新库，只需打印您已有的库即可：

myLib = Library()
myLib.addExistingBook(myBook)
print(myLib)

Answer 2

我想你想改变

def __str__(self):
        for myBook in self.collection[myBook]:
            self.collection[myBook]
            self.collection[myBook] = self.author

到

def __str__(self):
        for myBook in self.collection.keys():
            self.collection[myBook]
            self.collection[myBook] = self.author