candide <candide@free.invalid> wrote:
> I realize that built-in types objects don't provide a __dict__
attribute
> and thereby i can't set an attribute to a such object, for instance
>
>
> >>> a=[42,421]
> >>> a.foo="bar"
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> AttributeError: 'list' object has no attribute 'foo'
> >>> a.__dict__
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> AttributeError: 'list' object has no attribute '__dict__'
> >>>
>
>
> So, i was wondering :
>
> -- why this behaviour ?
Types without a __dict__ use less memory. Also, if you couldn't have a
type that didn't have a `__dict__` then any `dict` would also need its
own `__dict__` which would either result in infinite memory use or
recursive dictionaries.
It isn't just built-in types, you can choose for any type you define
whether or not to have a '__dict__' attribute
>>> class Fixed(object):
__slots__ = ('foo', 'bar')
readonly = 42
>>> f = Fixed()
>>> f.foo, f.bar = 1, 2
>>> f.foo, f.bar, f.readonly
(1, 2, 42)
>>> f.readonly = 24
Traceback (most recent call last):
File "<pyshell#31>", line 1, in <module>
f.readonly = 24
AttributeError: 'Fixed' object attribute 'readonly' is read-only
>>> f.baz = 'whatever'
Traceback (most recent call last):
File "<pyshell#32>", line 1, in <module>
f.baz = 'whatever'
AttributeError: 'Fixed' object has no attribute 'baz'
> -- where the official documentation refers to this point ?
>
See http://docs.python.org/reference/datamodel.html for the docs about
__slots__
There is also the API documentation which describes at a low level how
to control whether or not instances have a dict:
http://docs.python.org/c-api/typeobj.html#tp_dictoffset
I'm not sure though where you find a higher level statement of which
builtin types have a __dict__.
--
Duncan Booth http://kupuguy.blogspot.com
--
http://mail.python.org/mailman/listinfo/python-list
