26.1.3. Attribute references in old-style classes

The names inside the instance are called attributes. (The methods are technically attributes—attributes that are of type function.) There are three operations on attributes: get, set, and delete.

To get an attribute means to retrieve its value. Python searches the instance's .__dict__ for the attribute's name; if that name is found, its value is returned. If the instance's .__dict__ does not have a binding for the given name, Python searches the class's .__dict__. If no value is found there, Python searches the namespaces of the ancestor classes (if any).
```
>>> class C:
...     def __init__(self, x):
...         self.thingy = x
... 
>>> c=C(42)
>>> c.thingy
42
>>> c.__dict__['thingy']
42
```
When you call a method M of an instance I in an expression of the form “I.M(...)”, this is considered just another attribute “get” operation: the get operation I.M retrieves the method, and then that method is called using the arguments inside the “(...)”.
To set an attribute means to give it a value. If there is an existing attribute with the same name, its old value is discarded, and the attribute name is bound to the new value. The new value is stored in the instance's .__dict__.
```
>>> c.thingy
42
>>> c.thingy = 58
>>> c.thingy
58
>>> c.__dict__['thingy']
58
```

You can delete an attribute from an instance using a del statement (see Section 22.3, “The del statement: Delete a name or part of a value”).

>>> c.thingy
58
>>> del c.thingy
>>> c.thingy
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: C instance has no attribute 'thingy'

In addition to ordinary attributes and methods, your class can accept references to names that do not exist in the instance or class namespace. You can define special methods that will be called when some statement tries to get, set, or delete an attribute that isn't found in the instance's .__dict__. See Section 26.3.14, “__getattr__(): Handle a reference to an unknown attribute”, Section 26.3.21, “__setattr__(): Intercept all attribute changes”, and Section 26.3.9, “__delattr__(): Delete an attribute”.

If all else fails—if an attribute is not found in the instance's namespace and the class does not provide a special method that handles the attribute reference—Python will raise an AttributeError exception.

Python 2.7 quick reference

26.1.3. Attribute references in old-style classes