Which python classes allow assignment to not-yet-existent attributes, and which don't? [duplicate]

Question

So, I was playing around with Python while answering this question, and I discovered that this is not valid:

o = object()
o.attr = 'hello'

due to an AttributeError: 'object' object has no attribute 'attr'. However, with any class inherited from object, it is valid:

class Sub(object):
    pass

s = Sub()
s.attr = 'hello'

Printing s.attr displays 'hello' as expected. Why is this the case? What in the Python language specification specifies that you can't assign attributes to vanilla objects?

Answer 1

To support arbitrary attribute assignment, an object needs a __dict__: a dict associated with the object, where arbitrary attributes can be stored. Otherwise, there's nowhere to put new attributes.

An instance of object does not carry around a __dict__ -- if it did, before the horrible circular dependence problem (since dict, like most everything else, inherits from object;-), this would saddle every object in Python with a dict, which would mean an overhead of many bytes per object that currently doesn't have or need a dict (essentially, all objects that don't have arbitrarily assignable attributes don't have or need a dict).

For example, using the excellent pympler project (you can get it via svn from here), we can do some measurements...:

>>> from pympler import asizeof
>>> asizeof.asizeof({})
144
>>> asizeof.asizeof(23)
16

You wouldn't want every int to take up 144 bytes instead of just 16, right?-)

Now, when you make a class (inheriting from whatever), things change...:

>>> class dint(int): pass
... 
>>> asizeof.asizeof(dint(23))
184

...the __dict__ is now added (plus, a little more overhead) -- so a dint instance can have arbitrary attributes, but you pay quite a space cost for that flexibility.

So what if you wanted ints with just one extra attribute foobar...? It's a rare need, but Python does offer a special mechanism for the purpose...

>>> class fint(int):
...   __slots__ = 'foobar',
...   def __init__(self, x): self.foobar=x+100
... 
>>> asizeof.asizeof(fint(23))
80

...not quite as tiny as an int, mind you! (or even the two ints, one the self and one the self.foobar -- the second one can be reassigned), but surely much better than a dint.

When the class has the __slots__ special attribute (a sequence of strings), then the class statement (more precisely, the default metaclass, type) does not equip every instance of that class with a __dict__ (and therefore the ability to have arbitrary attributes), just a finite, rigid set of "slots" (basically places which can each hold one reference to some object) with the given names.

In exchange for the lost flexibility, you gain a lot of bytes per instance (probably meaningful only if you have zillions of instances gallivanting around, but, there are use cases for that).

Answer 2

As other answerers have said, an object does not have a __dict__. object is the base class of all types, including int or str. Thus whatever is provided by object will be a burden to them as well. Even something as simple as an optional __dict__ would need an extra pointer for each value; this would waste additional 4-8 bytes of memory for each object in the system, for a very limited utility.

---

Instead of doing an instance of a dummy class, in Python 3.3+, you can (and should) use types.SimpleNamespace for this.

Answer 3

It is simply due to optimization.

Dicts are relatively large.

>>> import sys
>>> sys.getsizeof((lambda:1).__dict__)
140

Most (maybe all) classes that are defined in C do not have a dict for optimization.

If you look at the source code you will see that there are many checks to see if the object has a dict or not.

Answer 4

So, investigating my own question, I discovered this about the Python language: you can inherit from things like int, and you see the same behaviour:

>>> class MyInt(int):
       pass

>>> x = MyInt()
>>> print x
0
>>> x.hello = 4
>>> print x.hello
4
>>> x = x + 1
>>> print x
1
>>> print x.hello
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
AttributeError: 'int' object has no attribute 'hello'

I assume the error at the end is because the add function returns an int, so I'd have to override functions like __add__ and such in order to retain my custom attributes. But this all now makes sense to me (I think), when I think of "object" like "int".

Answer 5

https://docs.python.org/3/library/functions.html#object :

Note: object does not have a __dict__, so you can’t assign arbitrary attributes to an instance of the object class.