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Re: object().__dict__

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  Re: object().__dict__ Cameron Simpson <cs@zip.com.au> - 2014-04-23 16:11 +1000

#70532 — Re: object().__dict__

On 23Apr2014 09:39, Pavel Volkov <sailor@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.

The base "object" class has a fixed set of attributes; you can't add more.

Almost every other class lets you add attributes, but the price for that is 
that it is slightly in memory footprint and slower to access.

Look up the "__slots__" dunder var in the Python doco index:

   https://docs.python.org/3/glossary.html#term-slots

You'll see it as a (rarely used, mostly discouraged) way to force a fixed set 
of attributes onto a class. As with object, this brings a smaller memory 
footprint and faster attribute access, but the price is flexibility.

Cheers,
Cameron Simpson <cs@zip.com.au>

Try being nothing but bored for 4 hours straight, and then tell me that
there's no fear involved.       - dave@elxr.jpl.nasa.gov (Dave Hayes)

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