![]() Now if we comply by providing the expected interface: class MyEffable(Effable): We are told that we haven't finished the job. TypeError: Can't instantiate abstract class MyEffable with abstract methods _str_ Now if we try to create an effable object without implementing the interface: class MyEffable(Effable):Īnd attempt to instantiate it: > MyEffable() Or in Python 3, with the slight change in metaclass declaration: class Effable(object, _str_(self): Raise NotImplementedError('users must define _str_ to use this base class') Say we wanted to define an abstract base class that is effable, in Python 2: class Effable(object): import abcįor example, "effable" is defined as something that can be expressed in words. The metaclass will be add the decorated functions to the _abstractmethods_ attribute, preventing instantiation until those are defined. ![]() We can create our own Abstract Base Class by setting the metaclass to abc.ABCMeta and using the abc.abstractmethod decorator on relevant methods. Implementation: Creating an Abstract Base Class """Alternate set implementation favoring space over speedĪnd not requiring the set elements to be hashable. Let's use this implementation example from the documentation: class ListBasedSet(collections.Set): So we are required to implement at least _contains_, _iter_, and _len_. TypeError: Can't instantiate abstract class MySet with abstract methods If we try to use it, we get an TypeError because the class we created does not support the expected behavior of sets: > MySet() Using an Abstract Base Classįor example, say we want to use one of the abstract base classes from the collections module: import collections In Python, we can use an abstract base class to define and enforce an interface. What is the difference between abstract class and interface in Python?Īn interface, for an object, is a set of methods and attributes on that object. This uses a kind of abstract superclass with mixins to create concrete subclasses that are disjoint. Pass # lots of stuff - but missing somethingĬlass Concrete1(SomeAbstraction, Mixin1):Ĭlass Concrete2(SomeAbstraction, Mixin2): Java uses interfaces because it doesn't have multiple inheritance.īecause Python has multiple inheritance, you may also see something like this class SomeAbstraction: The only differences would be in the stated intent in the docstring.Īnd the difference between abstract and interface is a hairsplitting thing when you have duck typing. If someone goes through the effort to define a formal interface, it will also be an abstract class. Raise NotImplementedError("Should have implemented this")īecause Python doesn't have (and doesn't need) a formal Interface contract, the Java-style distinction between abstraction and interface doesn't exist. Often listing the methods you're expected to supply.""" """Some description that tells you it's abstract, What you'll see sometimes is the following: class Abstract1:
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