public interface IInterface
{
void show();
}
public class MyClass : IInterface
{
#region IInterface Members
public void show()
{
Console.WriteLine("Hello World!");
}
#endregion
}
How do I implement Python equivalent of this C# code ?
class IInterface(object):
def __init__(self):
pass
def show(self):
raise Exception("NotImplementedException")
class MyClass(IInterface):
def __init__(self):
IInterface.__init__(self)
def show(self):
print 'Hello World!'
Is this a good idea?? Please give examples in your answers.
No, python does not have any equivalent of interfaces . Since Python does support multiple inheritance, you can easily emulate the equivalence of interfaces.
Unfortunately, Python doesn't have interfaces, or at least, not quite built into the language. Enter Python's abstract base class, or, cutely, ABC. Functionally, abstract base classes let you define a class with abstract methods, which all subclasses must implement in order to be initialized.
C vs Python languages are similar yet have many key differences. These languages are useful languages to develop various applications. The difference between C and Python is that Python is a multi-paradigm language and C is a structured programming language.
As mentioned by other here:
Interfaces are not necessary in Python. This is because Python has proper multiple inheritance, and also ducktyping, which means that the places where you must have interfaces in Java, you don't have to have them in Python.
That said, there are still several uses for interfaces. Some of them are covered by Pythons Abstract Base Classes, introduced in Python 2.6. They are useful, if you want to make base classes that cannot be instantiated, but provide a specific interface or part of an implementation.
Another usage is if you somehow want to specify that an object implements a specific interface, and you can use ABC's for that too by subclassing from them. Another way is zope.interface, a module that is a part of the Zope Component Architecture, a really awesomely cool component framework. Here you don't subclass from the interfaces, but instead mark classes (or even instances) as implementing an interface. This can also be used to look up components from a component registry. Supercool!
Using the abc module for abstract base classes seems to do the trick.
from abc import ABCMeta, abstractmethod
class IInterface:
__metaclass__ = ABCMeta
@classmethod
def version(self): return "1.0"
@abstractmethod
def show(self): raise NotImplementedError
class MyServer(IInterface):
def show(self):
print 'Hello, World 2!'
class MyBadServer(object):
def show(self):
print 'Damn you, world!'
class MyClient(object):
def __init__(self, server):
if not isinstance(server, IInterface): raise Exception('Bad interface')
if not IInterface.version() == '1.0': raise Exception('Bad revision')
self._server = server
def client_show(self):
self._server.show()
# This call will fail with an exception
try:
x = MyClient(MyBadServer)
except Exception as exc:
print 'Failed as it should!'
# This will pass with glory
MyClient(MyServer()).client_show()
Implementing interfaces with abstract base classes is much simpler in modern Python 3 and they serve a purpose as an interface contract for plug-in extensions.
Create the interface/abstract base class:
from abc import ABC, abstractmethod
class AccountingSystem(ABC):
@abstractmethod
def create_purchase_invoice(self, purchase):
pass
@abstractmethod
def create_sale_invoice(self, sale):
log.debug('Creating sale invoice', sale)
Create a normal subclass and override all abstract methods:
class GizmoAccountingSystem(AccountingSystem):
def create_purchase_invoice(self, purchase):
submit_to_gizmo_purchase_service(purchase)
def create_sale_invoice(self, sale):
super().create_sale_invoice(sale)
submit_to_gizmo_sale_service(sale)
You can optionally have common implementation in the abstract methods as in create_sale_invoice()
, calling it with super()
explicitly in the subclass as above.
Instantiation of a subclass that does not implement all the abstract methods fails:
class IncompleteAccountingSystem(AccountingSystem):
pass
>>> accounting = IncompleteAccountingSystem()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class IncompleteAccountingSystem with abstract methods
create_purchase_invoice, create_sale_invoice
You can also have abstract properties, static and class methods by combining corresponding annotations with @abstractmethod
.
Abstract base classes are great for implementing plugin-based systems. All imported subclasses of a class are accessible via __subclasses__()
, so if you load all classes from a plugin directory with importlib.import_module()
and if they subclass the base class, you have direct access to them via __subclasses__()
and you can be sure that the interface contract is enforced for all of them during instantiation.
Here's the plugin loading implementation for the AccountingSystem
example above:
...
from importlib import import_module
class AccountingSystem(ABC):
...
_instance = None
@classmethod
def instance(cls):
if not cls._instance:
module_name = settings.ACCOUNTING_SYSTEM_MODULE_NAME
import_module(module_name)
subclasses = cls.__subclasses__()
if len(subclasses) > 1:
raise InvalidAccountingSystemError('More than one '
f'accounting module: {subclasses}')
if not subclasses or module_name not in str(subclasses[0]):
raise InvalidAccountingSystemError('Accounting module '
f'{module_name} does not exist or does not '
'subclass AccountingSystem')
cls._instance = subclasses[0]()
return cls._instance
Then you can access the accounting system plugin object through the AccountingSystem
class:
>>> accountingsystem = AccountingSystem.instance()
(Inspired by this PyMOTW-3 post.)
interface supports Python 2.7 and Python 3.4+.
To install interface you have to
pip install python-interface
Example Code:
from interface import implements, Interface
class MyInterface(Interface):
def method1(self, x):
pass
def method2(self, x, y):
pass
class MyClass(implements(MyInterface)):
def method1(self, x):
return x * 2
def method2(self, x, y):
return x + y
I invite you to explore what Python 3.8 has to offer for the subject matter in form of Structural subtyping (static duck typing)
(PEP 544)
See the short description https://docs.python.org/3/library/typing.html#typing.Protocol
For the simple example here it goes like this:
from typing import Protocol
class MyShowProto(Protocol):
def show(self):
...
class MyClass:
def show(self):
print('Hello World!')
class MyOtherClass:
pass
def foo(o: MyShowProto):
return o.show()
foo(MyClass()) # ok
foo(MyOtherClass()) # fails
foo(MyOtherClass())
will fail static type checks:
$ mypy proto-experiment.py
proto-experiment.py:21: error: Argument 1 to "foo" has incompatible type "MyOtherClass"; expected "MyShowProto"
Found 1 error in 1 file (checked 1 source file)
In addition, you can specify the base class explicitly, for instance:
class MyOtherClass(MyShowProto):
but note that this makes methods of the base class actually available on the subclass, and thus the static checker will not report that a method definition is missing on the MyOtherClass
.
So in this case, in order to get a useful type-checking, all the methods that we want to be explicitly implemented should be decorated with @abstractmethod
:
from typing import Protocol
from abc import abstractmethod
class MyShowProto(Protocol):
@abstractmethod
def show(self): raise NotImplementedError
class MyOtherClass(MyShowProto):
pass
MyOtherClass() # error in type checker
There are third-party implementations of interfaces for Python (most popular is Zope's, also used in Twisted), but more commonly Python coders prefer to use the richer concept known as an "Abstract Base Class" (ABC), which combines an interface with the possibility of having some implementation aspects there too. ABCs are particularly well supported in Python 2.6 and later, see the PEP, but even in earlier versions of Python they're normally seen as "the way to go" -- just define a class some of whose methods raise NotImplementedError
so that subclasses will be on notice that they'd better override those methods!-)
Something like this (might not work as I don't have Python around):
class IInterface:
def show(self): raise NotImplementedError
class MyClass(IInterface):
def show(self): print "Hello World!"
My understanding is that interfaces are not that necessary in dynamic languages like Python. In Java (or C++ with its abstract base class) interfaces are means for ensuring that e.g. you're passing the right parameter, able to perform set of tasks.
E.g. if you have observer and observable, observable is interested in subscribing objects that supports IObserver interface, which in turn has notify
action. This is checked at compile time.
In Python, there is no such thing as compile time
and method lookups are performed at runtime. Moreover, one can override lookup with __getattr__() or __getattribute__() magic methods. In other words, you can pass, as observer, any object that can return callable on accessing notify
attribute.
This leads me to the conclusion, that interfaces in Python do exist - it's just their enforcement is postponed to the moment in which they are actually used
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