What you'll see sometimes is the following:

class Abstract1:
    """Some description that tells you it's abstract,
    often listing the methods you're expected to supply."""

    def aMethod(self):
        raise NotImplementedError("Should have implemented this")

Because Python doesn't have (and doesn't need) a formal Interface contract, the Java-style distinction between abstraction and interface doesn't exist. If someone goes through the effort to define a formal interface, it will also be an abstract class. The only differences would be in the stated intent in the docstring.

And the difference between abstract and interface is a hairsplitting thing when you have duck typing.

Java uses interfaces because it doesn't have multiple inheritance.

Because Python has multiple inheritance, you may also see something like this

class SomeAbstraction:
    pass  # lots of stuff - but missing something

class Mixin1:
    def something(self):
        pass  # one implementation

class Mixin2:
    def something(self):
        pass  # another

class Concrete1(SomeAbstraction, Mixin1):
    pass

class Concrete2(SomeAbstraction, Mixin2):
    pass

This uses a kind of abstract superclass with mixins to create concrete subclasses that are disjoint.

Python has duck typing :

If it walks like a duck and it quacks like a duck, then it must be a duck

Which means that this works :

class Duck:
    def quack(self):
        print("quack!")


class Car:
    def quack(self):
        print("vroum!")  # for example purposes


def make_it_quack(something):
    something.quack()  # don't care what it is, just that it quacks


make_it_quack(Duck())  # quack!
make_it_quack(Car())  # vroum!

Python at runtime will try to call the quack method on your object, and fails if it does not exist. But as long as the method exist (and the signature fit), Python does not care of the actual type of the object.

In other languages, you would have needed to define a base class (possibly abstract) that both child classes would have to inherit. And if there are multiple different sets of methods you want your object to have, either you 1) can do multiple-inheritance, 2) you build tall hierarchies of (abstract) base classes, or 3) you create lots of interfaces. Python does not need any of this, but it does not provide you static guarantees that your code type checks either.

If you want, there exist some ways to have more type checks in Python :

• you can declare abstract base classes using the standard library abc
• you can add type annotations to your code and use an external type checker (for exemple MyPy).

TL;DR : Python is duck-typed, but there is tooling if you want the guarantees that come with interface contracts.

Interfaces

An interface is a contract: The person writing the interface says, "hey, I accept things looking that way", and the person using the interface says "OK, the class I write looks that way".

An interface is an empty shell. There are only the signatures of the methods, which implies that the methods do not have a body. The interface can't do anything. It's just a pattern.

For example (pseudo code):

// I say all motor vehicles should look like this:
interface MotorVehicle
{
    void run();

    int getFuel();
}

// My team mate complies and writes vehicle looking that way
class Car implements MotorVehicle
{

    int fuel;

    void run()
    {
        print("Wrroooooooom");
    }


    int getFuel()
    {
        return this.fuel;
    }
}

Implementing an interface consumes very little CPU, because it's not a class, just a bunch of names, and therefore there isn't any expensive look-up to do. It's great when it matters, such as in embedded devices.

Abstract classes

Abstract classes, unlike interfaces, are classes. They are more expensive to use, because there is a look-up to do when you inherit from them.

Abstract classes look a lot like interfaces, but they have something more: You can define a behavior for them. It's more about a person saying, "these classes should look like that, and they have that in common, so fill in the blanks!".

For example:

// I say all motor vehicles should look like this:
abstract class MotorVehicle
{

    int fuel;

    // They ALL have fuel, so lets implement this for everybody.
    int getFuel()
    {
         return this.fuel;
    }

    // That can be very different, force them to provide their
    // own implementation.
    abstract void run();
}

// My teammate complies and writes vehicle looking that way
class Car extends MotorVehicle
{
    void run()
    {
        print("Wrroooooooom");
    }
}

Implementation

While abstract classes and interfaces are supposed to be different concepts, the implementations make that statement sometimes untrue. Sometimes, they are not even what you think they are.

In Java, this rule is strongly enforced, while in PHP, interfaces are abstract classes with no method declared.

In Python, abstract classes are more a programming trick you can get from the ABC module and is actually using metaclasses, and therefore classes. And interfaces are more related to duck typing in this language and it's a mix between conventions and special methods that call descriptors (the __method__ methods).

As usual with programming, there is theory, practice, and practice in another language