If you already use Django and want to expose an API, DRF is a natural choice. Flask is more simple to use, if you have to do simple things. If you need models,

If you already use Django and want to expose an API, DRF is a natural choice. Flask is more simple to use, if you have to do simple things. If you need models, access to a database and you don't want to pull third party libraries for things that are normally part of a complete framework, I usually suggest to use Django rather then Flask. This said, Flask remains a nice, light framework and I personally use it for a few services.