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Guillaume Crt
6,395 Pointsusing __init__ method instead of class attribute in this example ?
in the example this code is used :
{
class Bookcase:
def __init__(self):
self.books = []
}
but I think writing that is more logical :
{
class Bookcase:
books = []
}
Am I wrong ? Is there any major difference ? Thanks
2 Answers
Chris Freeman
Treehouse Moderator 68,457 PointsGreat question @Guilla Guillaume Crt!! The issue is that the attribute books is a mutable list.
As a class variable, each instance would be operating on the same list. If created in the __init__ method, the books list would be local to each instance and independent of the other instances.
Look at the following code to see this in action:
$ python
Python 3.8.5 (default, Jul 28 2020, 12:59:40)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class Bookcase1:
... def __init__(self):
... self.books = []
...
>>> class Bookcase2:
... books = []
...
# playing with instances using __init__
>>> bc1a = Bookcase1()
>>> bc1a.books
[]
>>> bc1a.books.append('Title 1a')
>>> bc1a.books
['Title 1a']
>>> bc1b = Bookcase1()
>>> bc1b.books
[]
>>> bc1b.books.append('Title 1b')
>>> bc1b.books
['Title 1b']
>>> bc1a.books
['Title 1a']
# playing with instances using class attribute
>>> bc2a = Bookcase2()
>>> bc2a.books
[]
>>> bc2a.books.append('Title 2a')
>>> bc2a.books
['Title 2a']
>>> bc2b = Bookcase2()
# notice the second instance is operating on the same list
>>> bc2b.books
['Title 2a']
>>> bc2b.books.append('TItle 2b')
>>> bc2b.books
['Title 2a', 'TItle 2b']
>>> bc2a.books
['Title 2a', 'TItle 2b']
# They are literally the same object.
>>> bc2a.books is bc2b.books
True
Going beyond your question, if using a class attribute, a local attribute can be forced:
>>> bc2b.books = []
# bc2b.books is now local
>>> bc2b.books
[]
>>> bc2b.books.append('New Title 2b')
>>> bc2b.books
['New Title 2b']
>>> bc2a.books
['Title 2a', 'TItle 2b']
# bc2a still using class attribute
# if local bc2b.books is deleted,
# the attribute reference reverts to class attribute
>>> del bc2b.books
>>> bc2b.books
['Title 2a', 'TItle 2b']
Post back if you need more help. Good Luck!!
Guillaume Crt
6,395 PointsI get it , thank you very much !