问题
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__().
On the contrary, I get this:
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!
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=[])
?
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?
回答1:
You should not use [] as default argument.
class Book(object):
def __init__(self, title=None, book_rows=None):
self.title = title
self.book_rows = book_rows or []
来源：https://stackoverflow.com/questions/31453008/new-instance-has-old-instance-attribute-values