.. include:: global.rst

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14: Class and  Type
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Summary
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- There are two main concepts of type
   - Primitive type: relates to the way data is represented and is the origin of the concept
   - The second relates to class in object oriented programming
- In many object oriented languages defining a class introduces a new type to the type system
- A strongly typed language enforces the rule that you have to declare the type of a variable and that variable can from then on only reference objects of that type
- This rule is usually extended to include variables referencing objects of the specified type or a subtype
   - The reason for this si that it allows you to write partly generic methods which can process the type and all its subtypes
- Class hierarchy is a model of the real world
- The Liskov Substitution Principle is often used as a justification for hierarchial typing, but it is only an approximation to the real world
- Variables do not have a type associated with them and can reference any type of object
- Objects do have a limited notion of type in that their __class__ attribute is set to the class or metaclass that created them
   - It is important to realize that __class__ can be modified
- You can use isinstance and issubclass to check that an object claims to be of the appropriate type
- Another approach is to use defensive programming to test for the presence of any attribute or method you are planning to use via the hasattr function
   - This is generally called "duck typing"




Program
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.. literalinclude:: programs/chapterFourteen.py
   :language: python

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

.. code-block:: console