It's a thing that bugged me for a while. Why can't I do:
>>> a = ""
>>> a.foo = 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'foo'
...while I can do the following?
>>> class Bar():
... pass
...
>>> a = Bar()
>>> a.foo = 10 #ok!
What's the rule here? Could you please point me to some description?
You can add attributes to any object that has a __dict__.
x = object() doesn't have it, for example.__slots__ also do not have it.class have it unless the previous statement applies.If an object is using __slots__ / doesn't have a __dict__, it's usually to save space. For example, in a str it would be overkill to have a dict - imagine the amount of bloat for a very short string.
If you want to test if a given object has a __dict__, you can use hasattr(obj, '__dict__').
This might also be interesting to read:
Some objects, such as built-in types and their instances (lists, tuples, etc.) do not have a
__dict__. Consequently user-defined attributes cannot be set on them.
Another interesting article about Python's data model including __dict__, __slots__, etc. is this from the python reference.
class Bar(object):instead ofclass Bar():so you get a new-style class