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Re: can't add variables to instances of built-in classes

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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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Thread

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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