Why don't we require interfaces in dynamic languages?

Question

Is it just because of dynamic typing we don't require a concept of interfaces(like in Java and C#) in python?

Answer 1

The interface as a keyword and artifact was introduced by Java¹ ( and C# took it from there ) to describe what the contract an object must adhere was.

But, interface has always been a key part of Object Oriented Paradigm and basically it represents the methods an object has to respond. Java just enforces this mechanism to provide statically type checking.

So, dynamic ( OO ) programming languages do use interfaces, even thought they don't statically check them. Just like other data types, for instance in Ruby:

 @i = 1;

You don't have to declare i of type FixNum you just use it. Same goes for interfaces, they just flow. The trade-off is, you can't have a static check on that and failures are only show at runtime.

In the other hand Structural type ( or static duck type as I call it :P ) used by languages as Go or Scala, gives the best of both worlds.

_{1. See Daniel Earwicker comment about CORBA interface keyword}

Answer 2

We don't require them, but we do support them. Check out Zope Interfaces (which can be and are used outside of Zope).

Answer 3

It's worth noting that, contrary to what many people will say as a first response, interfaces can be used to do more than document "what methods a class supports". Grzenio touches on this with his wording on "implement the same behaviour". As a specific example of this, look at the Java interface Serializable. It doesn't implement any methods; rather it's used as a "marker" to indicate that the class can be serialized safely.

When considered this way, it could be reasonable to have a dynamic language that uses interfaces. That being said, something akin to annotations might be a more reasonable approach.

Answer 4

Interfaces are used in statically typed languages to describe that two otherwise independent objects "implement the same behaviour". In dynamically typed languages one implicitly assumes that when two objects have a method with the same name/params it does the same thing, so interfaces are of no use.