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Groups > comp.lang.python > #111570
| From | Peter Otten <__peter__@web.de> |
|---|---|
| Newsgroups | comp.lang.python |
| Subject | Re: can't add variables to instances of built-in classes |
| Date | 2016-07-17 13:40 +0200 |
| Organization | None |
| Message-ID | <mailman.63.1468755660.2307.python-list@python.org> (permalink) |
| References | <4f796d95-0e87-4610-a0a8-1eff07c60ace@googlegroups.com> <nmfqrg$ea7$1@ger.gmane.org> |
Kent Tong wrote:
> Hi,
>
> I can add new variables to user-defined classes like:
>
>>>> class Test:
> ... pass
> ...
>>>> a=Test()
>>>> a.x=100
>
> but it doesn't work if the instances belong to a built-in class such as
> str or list:
>
>>>> a='abc'
>>>> a.x=100
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> AttributeError: 'str' object has no attribute 'x'
>
> What makes this difference?
By default custom classes have a dictionary (called __dict__) to hold these
attributes. If for every string or integer there were such a dict that would
waste a lot of memory. You can subclass if you need it:
>>> class Str(str): pass
...
>>> s = Str("hello")
>>> s.x = 42
>>> s
'hello'
>>> s.x
42
You can also avoid the dict in your own classes by specifiying slots for
allowed attributes:
>>> class Test:
... __slots__ = ("foo", "bar")
...
>>> t = Test()
>>> t.foo = 42
>>> t.baz = "whatever"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Test' object has no attribute 'baz'
Use this feature sparingly, only when you know that there are going to be
many (millions rather than thousands) of Test instances.
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can't add variables to instances of built-in classes Kent Tong <kent.tong.mo@gmail.com> - 2016-07-17 03:57 -0700
Re: can't add variables to instances of built-in classes Peter Otten <__peter__@web.de> - 2016-07-17 13:40 +0200
Re: can't add variables to instances of built-in classes Wilson Ong <wilsonokw@gmail.com> - 2016-07-17 04:50 -0700
Re: can't add variables to instances of built-in classes Chris Angelico <rosuav@gmail.com> - 2016-07-17 22:02 +1000
Re: can't add variables to instances of built-in classes Steven D'Aprano <steve@pearwood.info> - 2016-07-18 01:04 +1000
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-17 20:00 -0700
Re: can't add variables to instances of built-in classes Peter Otten <__peter__@web.de> - 2016-07-18 09:38 +0200
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-18 14:48 -0700
Re: can't add variables to instances of built-in classes breamoreboy@gmail.com - 2016-07-18 16:12 -0700
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-18 19:53 -0700
Re: can't add variables to instances of built-in classes breamoreboy@gmail.com - 2016-07-19 14:24 -0700
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-19 16:58 -0700
Re: can't add variables to instances of built-in classes Chris Angelico <rosuav@gmail.com> - 2016-07-20 16:19 +1000
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-19 23:45 -0700
Re: can't add variables to instances of built-in classes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-07-20 17:22 +1000
Re: can't add variables to instances of built-in classes Peter Otten <__peter__@web.de> - 2016-07-20 09:26 +0200
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-20 01:50 -0700
Re: can't add variables to instances of built-in classes Peter Otten <__peter__@web.de> - 2016-07-20 11:15 +0200
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-20 15:11 -0700
Re: can't add variables to instances of built-in classes Chris Angelico <rosuav@gmail.com> - 2016-07-21 12:29 +1000
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-08-03 19:34 -0700
Re: can't add variables to instances of built-in classes Chris Angelico <rosuav@gmail.com> - 2016-08-04 13:00 +1000
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-08-04 16:57 -0700
Re: can't add variables to instances of built-in classes Steven D'Aprano <steve@pearwood.info> - 2016-07-21 12:48 +1000
Re: can't add variables to instances of built-in classes Steven D'Aprano <steve@pearwood.info> - 2016-07-20 22:10 +1000
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-20 14:48 -0700
Re: can't add variables to instances of built-in classes breamoreboy@gmail.com - 2016-07-20 18:04 -0700
Re: can't add variables to instances of built-in classes Peter Otten <__peter__@web.de> - 2016-07-21 08:59 +0200
Re: can't add variables to instances of built-in classes Lawrence D’Oliveiro <lawrencedo99@gmail.com> - 2016-07-20 02:10 -0700
Re: can't add variables to instances of built-in classes Ethan Furman <ethan@stoneleaf.us> - 2016-07-18 06:10 -0700
Re: can't add variables to instances of built-in classes Kent Tong <kent.tong.mo@gmail.com> - 2016-07-17 04:53 -0700
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