Latest recipes tagged "inheritance"

How to use super() effectively -- Python 3.2 version (Python)

2015-12-01T16:23:09-08:00

Python recipe 579130 by Lance E Sloan (cooperative, inheritance, multiple, next_in_mro, super).

Python's super() provides a unique and amazing capability. It allows subclasses to be written to reorder a chain method calls. The recipe demonstrates all of the tradecraft needed to get super() to do your bidding.

How to use super() effectively -- Python 2.7 version (Python)

2015-12-01T16:20:26-08:00

Python recipe 579129 by Lance E Sloan (cooperative, inheritance, multiple, next_in_mro, super).

Inherit method docstrings without breaking decorators or violating DRY (Python)

2013-07-01T02:29:40-07:00

Python recipe 578587 by nikratio (decorator, decorators, docstring, inheritance). Revision 2.

There are several recipes for inheriting method docstrings. However, most of them either violate DRY (and have you repeat the ancestor class name in the method decorator that sets the docstring), or do break decorators that try to access the docstring (because the docstring is only assigned after class creation). This recipe avoids both problems.

Note: This recipe uses the mro method from http://code.activestate.com/recipes/577748-calculate-the-mro-of-a-class/

CLOS-like around/before/after auxiliary methods (Python)

2011-08-25T22:59:22-07:00

Python recipe 577859 by Jan Kaliszewski (auxiliary, class, clos, decorator, inheritance, method, object, super).

This module provides an easy way to define and use your own around/before/after auxiliary methods, similar to those used in CLOS (Common Lisp Object System).

Using Metaclasses and Class Decorators to Inherit Function Docstrings (Python)

2011-06-11T00:59:35-07:00

Python recipe 577743 by Eric Snow (decorator, decorators, docstring, inheritance, metaclass, metaclasses). Revision 4.

You'll find three different approaches to copying the method's docstring to the overriding method on a child class.

The function decorator approach is limited by the fact that you have to know the class when you call the decorator, so it can't be used inside a class body. However, recipe #577746 provides a function decorator that does not have this limitation.

Mixin and Overlay (Python)

2011-06-01T05:16:47-07:00

Python recipe 577730 by Eric Snow (classes, inheritance, mixins).

The Two Sides of Classes

Class inheritance either adds new attributes to a class, or changes existing attributes. However, this distinction is not made in Python's class system. This recipe is a stab at providing that separation. It takes advantage of metaclasses and class decorators to do it.

For this recipe the two sides of class inheritance are called mixins and overlays. Mixins add new attributes to a class. Overlays change existing attributes, which in the case of methods means changing the behavior of the methods. However, overlays do not add any attributes.

Separation of Concerns

Another issue with class inheritance is that abstraction rarely breaks down into perfect trees with a clear separation of concerns. Python allows multiple inheritance, which can help, but requires cooperation between classes in the diamond hierarchy. Mixins and overlays help with this problem. Hopefully, that will be evidenced by the recipe and subsequent examples.

With this approach, the main single inheritance line can focus on the core abstraction and mixins/overlays can be used to extend the classes in other directions. This does not solve all the problems regarding separation of concerns, but it solves some. I hope to address the rest in another recipe that centers around delegation through a component architecture on instances.

Interfaces

Since Python 2.6 we have had Abstract Base Classes providing a mechanism for promising what interfaces an object provides. I'll show in one of the examples how an ABC can be split into a interface portion and a mixin portion. The recipe takes advantage of the ABC functionality in the Python type system. I expect that mixins and overlays would be a good fit with one of my other recipes (Recipe 577711).

The Recipe Classes

This recipe provides a standard approach to applying mixin classes without using inheritance. This is done through a metaclass. The metaclass builds a __mixins__ attribute on the class and provides a mixes_in class decorator. Applying that decorator to the class will add the attributes in __mixins__ to the decorated class. However, if any of those names are already bound on the name then it will fail. The decorator returns the modified class.

Note: Traditional mixins are typically done through multiple inheritance, as opposed to class decorators.

This recipe also provides a companion to mixin classes, called overlays. This is done through a metaclass in exactly the same way as the mixins, but provides an __overlays__ attribute and a class decorator called overlays. In contrast to mixins, if an overlay attribute is missing on the decorated class, it will fail. This is because the decorator will return a new class that inherits from the decorated class, and overrides the attributes in __overlays__.

Note: Metaclasses are used here because we need to pull from the class namespace there. A class decorator does not afford us the same functionality without more complexity.

Note: This recipe should work fine in 2.7 with a switch to the __metaclass__ syntax.

How to use super() effectively -- Python 2.7 version (Python)

2011-06-01T19:19:37-07:00

Python recipe 577721 by Raymond Hettinger (cooperative, inheritance, multiple, next_in_mro, super). Revision 8.

How to use super() effectively (Python)

2011-06-01T19:17:58-07:00

Python recipe 577720 by Raymond Hettinger (cooperative, inheritance, multiple, next_in_mro, super). Revision 8.

Abstract method decorator (Python)

2011-04-20T21:50:23-07:00

Python recipe 577666 by jimmy2times (abstract, decorator, exception, inheritance, method, python, subclass). Revision 5.

A simple decorator that helps define abstract methods: when such a method is called, an appropriate exception is raised.

Composition of classes instead of multiple inheritance (Python)

2011-04-16T03:40:19-07:00

Python recipe 577658 by Ethan Furman (class_decorator, composition, inheritance, multiple_inheritance).

MI can be difficult and confusing, and if the base classes don't cooperate -- well, cooperative MI won't work.

One way around this is to use composition instead. This class decorator will combine the source classes with the target class, ensuring that no duplications occur (raises a TypeError if there are any).

namedtuple.abc - abstract base class + mix-in for named tuples (Python)

2011-04-02T02:07:00-07:00

Python recipe 577629 by Jan Kaliszewski (abc, collections, dry, inheritance, namedtuple, shortcuts). Revision 7.

If you need

to define named tuple subclasses (including reusable abstract ones), adding/overriding some methods, in a convenient way;
to have the named tuple ABC (abstract base class) for isinstance/issubclass tests;
or simply would like to define your named tuple classes in a class-syntax-based and DRY way (without repeating type names...)

-- this recipe is for you.

Bound Inner Classes, Using An Alternate Approach (Python)

2011-03-24T09:47:11-07:00

Python recipe 577623 by Larry Hastings (getattribute, inheritance). Revision 2.

An alternate implementation approach for bound inner classes. For a description of bound inner classes, see this recipe: http://code.activestate.com/recipes/577070-bound-inner-classes/

This recipe works in both Python 2.6 and 3.1.

Nested contexts -- a chain of mapping objects (Python)

2010-10-25T02:13:37-07:00

Python recipe 577434 by Raymond Hettinger (ast, chained, compiler, dictionary, inheritance, nested, nonlocal, scopes, xml). Revision 2.

Easy to use chain of dictionaries for crafting nested scopes or for a tree of scopes. Useful for analyzing AST nodes, XML nodes or other structures with multiple scopes. Can emulate various chaining styles including static/lexical scoping, dynamic scoping and Python's own globals(), locals(), nested scopes, and writeable nonlocals. Can also model Python's inheritance chains: instance dictionary, class dictionary, and base classes.

Docstring inheritance class decorator (Python)

2011-11-23T20:12:03-08:00

Python recipe 577102 by Alec Thomas (class, decorator, docstring, inheritance). Revision 2.

Inherits method docstrings from parent classes.

Design Pattern -- Inherit from an instance (Python)

2010-02-22T06:04:16-08:00

Python recipe 577026 by Daniel Cohn (design_pattern, factory_class, inheritance, instance). Revision 4.

Take an instance (call it foo) and create a factory class (call it InstanceFactory) that produces foo's. Then inherit from InstanceFactory.

Find all subclasses of a given class (Python)

2009-11-04T20:26:08-08:00

Python recipe 576949 by Gabriel Genellina (class, extending, inheritance, plugin, subclass, subclasses, type). Revision 3.

itersubclasses(cls) returns a generator over all subclasses of cls, in depth first order. cls must be a new-style class; old-style classes are not supported.

Docstring inheritance decorator (Python)

2009-07-28T13:42:32-07:00

Python recipe 576862 by Shai Berger (decorator, descriptor, docstring, inheritance, metaprogramming).

In many cases, a subclass overrides a method in a parent class, just to change its implementation; in such cases, it would be nice to preserve the overridden method's docstring. The decorator below can be used to achieve this without explicit reference to the parent class. It does this by replacing the function with a descriptor, which accesses the parent class when the method is accessed as an attribute.

SuperFly: Separating class heirarchy from class definition. (Python)

2009-02-15T20:47:51-08:00

Python recipe 576652 by Ted Skolnick (inheritance, polymorphism). Revision 2.

This is a recipe for defining class hierarchies outside of class definitions. This way you could at, runtime, decide what class should act as the parent of a given class. A class could sometimes have one parent, sometimes another.