新实例具有旧实例属性值 [英] New instance has old instance attribute values

问题描述

我想实例化书籍的多个实例。在我的 __ init __（）方法中，指定一些默认值。因此，我希望每次实例化一本新书时，所有旧值都将替换为我为 __ init __（）的参数指定的默认值。

I want to instantiate multiple instances of books. In my __init__() method I specify some default values. So I am expecting that every time I instantiate a new book, all the old values will be replaced by the default ones I specified as arguments for __init__().

相反，我明白了：

class Book(object):
    def __init__(self, title=None, book_rows=[]):
        self.title = title
        self.book_rows = book_rows
    def add_line(self, book_row):
        assert type(book_row) == BookLine
        self.book_rows.append(book_row)

class BookLine(object):
    def __init__(self, sent):
        self.sent = sent

foo = Book(title='First book')
book_row = BookLine(sent='This is a sentence')
foo.add_line(book_row)

foo.title
# 'First book'
foo.book_rows[0].sent
# 'This is a sentence'

foo = Book(title='Second book. This should be new')
foo.title
# 'Second book. This should be new' <-- OK

foo.book_rows[0].sent
# 'This is a sentence' <-- WRONG!

为什么 foo.book_rows [0] .sent 还在吗？自从我写道：我的 __ init __（）方法不是应该擦除所有 book_rows 的方法，

Why is foo.book_rows[0].sent still there? Isn't my __init__() method supposed to wipe all the book_rows, since I wrote:

__ init __（self，title = None，book_rows = []）

？

PS我知道有一个类似的问题，但这是关于类变量的。我认为这里的变量不是类变量。我错了吗？

P.S. I know there's a similar question, but it was about class variables. I think that my variables here are not class variables. Am I wrong?

推荐答案

您不应默认使用 []

class Book(object):
    def __init__(self, title=None, book_rows=None):
        self.title = title
        self.book_rows = book_rows or []

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