.. include:: global.rst

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11: Custom Attribute Access
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Summary
============

- The descriptor object can customize to any attribute via the __get__, __set__ and __delete__ functions which are automatically called on attribute access or deletion
- If a descriptor defines __get__ and __set__ then it is a data descriptor
    - If only __get__ is defined it is a non-data descriptor
- Data descriptors cannot be overridden by a definition in the instance, whereas non-data descriptors can be
- __getattribute__ is called for every attribute access and if you override it then you override every attribute access
- If __getattribute__ fails to find an attribute then it automatically calls __getattr__, if it is defined
- __setattr__ is called every time you attempt an assignment to an attribute
- You can use __getattr__ to implement default attributes and methods
- You can use del or delattr to delete an attribute, but in either case __delattr__ is called, if it is defined
- The __dir__ magic method can be used to define a custom dir command
- Slots are just a more efficient way of implementing attributes using a descriptor
- Another way of accessing attributes is via the index or key operator, []
    - This is customized using __getitem__ and __setitem__
    - There are also a range of addirtional magic methods that can make attribute access look more like a collection

Program
============

.. literalinclude:: programs/chapterEleven.py
   :language: python

Program Output
=================

.. code-block:: console


    [__setattr__] Setting slot_attr = I am in slots

    --- DATA DESCRIPTOR ---
    [__setattr__] Setting data = 10
    [DataDescriptor __set__] Setting data = 10
    [__getattribute__] Accessing: __dict__
    [__getattribute__] Accessing: data
    [DataDescriptor __get__] Getting data
    [__getattribute__] Accessing: __dict__
    10
    [__delattr__] Deleting data
    [DataDescriptor __delete__] Deleting data
    [__getattribute__] Accessing: __dict__

    --- NON-DATA DESCRIPTOR ---
    [__getattribute__] Accessing: non_data
    [NonDataDescriptor __get__] Accessed
    default
    [__setattr__] Setting non_data = override
    [__getattribute__] Accessing: non_data
    override

    --- __getattr__ FALLBACK ---
    [__getattribute__] Accessing: missing_attr
    [__getattribute__] Accessing: __getattr__
    [__getattr__] missing_attr not found, returning default
    default_missing_attr

    --- __setattr__ ---
    [__setattr__] Setting new_attr = 42

    --- __delattr__ ---
    [__delattr__] Deleting new_attr

    --- __dir__ ---
    ['custom_attr1', 'custom_attr2', 'data', 'non_data']

    --- __slots__ ---
    [__getattribute__] Accessing: slot_attr
    I am in slots

    --- COLLECTION ACCESS ---
    [__setitem__] Setting key: key = value
    [__getattribute__] Accessing: __dict__
    [__getitem__] Getting key: key
    [__getattribute__] Accessing: __dict__
    valuec