It’s been a while, but I’m pretty sure the reason why a dictionary can’t have a dictionary as it’s key is because the dictionary class doesn’t implement the __hash__ method that would compute the hashcode.

This is definitely the proximate reason.

The reason for that choice is because the object is mutable.

Sure, but as others have mentioned, you can make your own choice by subclassing dict and defining your own hash function, as long as that function is defined in a way that follows the appropriate rules.