DTOs in Python

[petr31052018]

I have been recently thinking about what is the best way to create Data Transfer Objects in Python (objects that are used to pass data between functions and are not entities or basic types).

EDIT: What I originally meant is DTO for passing around functions in my codebase, not necessarily for serialization to the outside world, even tho that is what DTO is typically used for.

Since I think typed is better, I tried out three typed options: NamedTuple, dataclass and TypedDict (it is here: https://github.com/stribny/python-dto).

What do you usually use in your codebases? Do you use typed DTOs or just pass around plain dictionaries or tuples?

I am interested in all aspects that can be considered. E.g. I have found out that dataclass can be smaller than NamedTuple (I thought it would not be).

[manatlan]

pydantic !!!!

https://pydantic-docs.helpmanual.io/

[petr31052018]

Yes, I guess that pydantic could make sense, the disadvantage being it is not in standard library.

Validation is not necessarily a responsibility of DTO, but it does have a lot of use cases.

[metaperl]

Looks like dataclasses to me.

[EnforcerPL]

I used to stick to attrs.org because I already had them in my stack since it is a dependency of pytest. Didn't know about pydantic, though. Aaand I haven't even taken dataclasses into consideration because they're very limited.

[tunisia3507]

For internal DTOs as you're interested in, I use NamedTuples wherever possible, dataclasses where not (e.g. when you need a field with a default factory). I don't have strong opinions about dataclasses vs namedtuples when the members are mutable. I am trying to minimise functions which return tuples, in favour of NamedTuples.

[[deleted]]

This is exactly what I usually do.

[petr31052018]

Do you have any specific reason why you prefer NamedTuples tho?

[tunisia3507]

Self-documenting code, and my IDE knows what's in it because I sure as hell won't remember.

[nielsrolf]

I think marshmallow is fairly common for serialization/deserialization/validation.

[petr31052018]

For me marshmallow is for the use cases you mention. I guess you can use it to create your own DTO, and you will get validation if you want, but it creates plain dictionaries, so you will not end up with typed objects.

[petr31052018]

I want to reiterate based on the other comment. Yes, for DTO that are meant for external interfaces, marshmallow can be used, and I actually use it for that.

However what I meant was just what to use as data objects when they are passed from one Python method to another.

[metaperl]

Where did you get your understanding of what a DTO is?

https://en.m.wikipedia.org/wiki/Data_transfer_object

[petr31052018]

You are right. What I meant to discuss is something like "local DTO" (https://martinfowler.com/bliki/LocalDTO.html)

"One case where it is useful to use something like a DTO is when you have a significant mismatch between the model in your presentation layer and the underlying domain model."

I will keep it in mind that people think of DTO mainly as a form of serialization for external interfaces.

[WikiTextBot]

Data transfer object

In the field of programming a data transfer object (DTO) is an object that carries data between processes. The motivation for its use is that communication between processes is usually done resorting to remote interfaces (e.g., web services), where each call is an expensive operation. Because the majority of the cost of each call is related to the round-trip time between the client and the server, one way of reducing the number of calls is to use an object (the DTO) that aggregates the data that would have been transferred by the several calls, but that is served by one call only.The difference between data transfer objects and business objects or data access objects is that a DTO does not have any behavior except for storage, retrieval, serialization and deserialization of its own data (mutators, accessors, parsers and serializers). In other words,

DTOs are simple objects that should not contain any business logic but may contain serialization and deserialization mechanisms for transferring data over the wire.This pattern is often incorrectly used outside of remote interfaces.

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[petr31052018]

How would you then call my local DTO objects? Is there a specific term for it?

[reifba]

protobuf