3.3.2 Customizing attribute access
The following methods can be defined to customize the meaning of attribute access (use
of, assignment to, or deletion of
x.name) for class instances.
- Called when an attribute lookup has not found the attribute in the usual places
(i.e. it is not an instance attribute nor is it found in the class tree for
name is the attribute name. This method should return the (computed)
attribute value or raise an AttributeError exception.
Note that if the attribute is found through the normal mechanism, __getattr__() is not called. (This is an intentional asymmetry
between __getattr__() and __setattr__().)
This is done both for efficiency reasons and because otherwise __setattr__()
would have no way to access other attributes of the instance. Note that at least for
instance variables, you can fake total control by not inserting any values in the
instance attribute dictionary (but instead inserting them in another object). See the __getattribute__() method below for a way to actually get total
control in new-style classes.
||self, name, value)
- Called when an attribute assignment is attempted. This is called instead of the
normal mechanism (i.e. store the value in the instance dictionary). name is
the attribute name, value is the value to be assigned to it.
If __setattr__() wants to assign to an instance attribute,
it should not simply execute "self.name = value"
-- this would cause a recursive call to itself. Instead, it should insert the value in
the dictionary of instance attributes, e.g., "self.__dict__[name]
= value". For new-style classes, rather than accessing the instance
dictionary, it should call the base class method with the same name, for example,
"object.__setattr__(self, name, value)".
- Like __setattr__() but for attribute deletion instead of
assignment. This should only be implemented if "del obj.name"
is meaningful for the object.