Entity vs Value
How do I Decide if Something is a Value or an Entity?
The question you should think about first when choosing how to represent something is: what happens when I write down a literal expression for this thing and were to "instantiate" it twice?
- should it refer to the identical "thing"?
- does it denote two different things of the same type?
If (1) is the answer, you're dealing with a value.
If (2) is the answer, you're dealing with something that has an identity, e.g. an entity.
I thought you said Zef has "value semantics" everywhere?
Yes, this is mostly true. It is also true for the "atoms", i.e. the concrete Python objects you are dealing with.
To resolve this paradox, we have to get slightly philosophical: Suppose you're dealing some entity that has value semantics, e.g. a "User" which you are storing in a database and dealing with in your code. The obvious, but fundamentally critical point you always need to remember is the following: your Python object representing this customer IS NOT that person. The actual person lives in the real world. All you have in your Python process or your database is something you choose to be a representation of that person. It is simply a "handle" or pointer to refer to whichever actual real-world entity or abstract concept you choose to represent.
- The actual entity / person has an identity and does not follow value semantics
- The handle / entity object in your system has value semantics. If you make a copy of it, it points to the real thing.
This is analogous to pointers in languages like C++ having value semantics. Even if they point to an object which does not have value semantics.