object().__dict_
Duncan Booth
duncan.booth at invalid.invalid
Wed Apr 23 04:56:07 EDT 2014
Pavel Volkov <sailor at lists.xtsubasa.org> wrote:
> There are some basics about Python objects I don't understand.
> Consider this snippet:
>
>>>> class X: pass
> ...
>>>> x = X()
>>>> dir(x)
> ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__',
> '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__',
> '__hash__', '__init__', '__le__', '__lt__', '__module__', '__ne__',
> '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__',
> '__sizeof__', '__str__', '__subclasshook__', '__weakref__']
>>>> x.foo = 11
>
> And now I want to make a simple object in a shorter way, without
> declaring X class:
>
>>>> y = object()
>>>> dir(y)
> ['__class__', '__delattr__', '__dir__', '__doc__', '__eq__',
> '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__',
> '__init__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__',
> '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
> '__subclasshook__']
>>>> y.foo = 12
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> AttributeError: 'object' object has no attribute 'foo'
>
> The attribute list is different now and there's no __dict__ and the
> object does not accept new attributes.
> Please explain what's going on.
>
>
Not all classes have a __dict__ attribute. Mostly builtin classes (e.g.
tuple, list, int, ...), but also if you have an class using __slots__
which subclasses a class with no __dict__ it won't have a __dict__.
Subclasses don't remove attributes, they only add them (although in
Python you can bend that rule it still applies here). Therefore for any
classes to not have a __dict__ attribute the ultimate base class
'object' has to not have a __dict__.
The consequence, as you found out, is that you cannot add attributes to
an instance of 'object()', you have to create at least an empty subclass
which doesn't include a `__slots__` attribute to get a class that can
accept arbitrary attributes.
--
Duncan Booth
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