Plurrrr

There are three different types of inheritance going on.

1. Ontological inheritance is about specialisation: this thing is a specific variety of that thing (a football is a sphere and it has this radius)
2. Abstract data type inheritance is about substitution: this thing behaves in all the ways that thing does and has this behaviour (this is the Liskov substitution principle)
3. Implementation inheritance is about code sharing: this thing takes some of the properties of that thing and overrides or augments them in this way. The inheritance in my post On Inheritance is this type and only this type of inheritance.

These are three different, and frequently irreconcilable, relationships. Requiring any, or even all, of them, presents no difficulty. However, requiring one mechanism support any two or more of them is asking for trouble.

Today I am going to introduce a new concept for Python developers: typeclasses. It is a concept behind our new dry-python library called classes.

I will tell you in advance, that it will look very familiar to what you already know and possibly even use. Moreover, we reuse a lot of existing code from Python’s standard library. So, you can call this approach “native” and “pythonic”. And it is still going to be interesting: I am showing examples in 4 different languages!

But, before discussing typeclasses themselves, let’s discuss what problem they do solve.