For example, I have a post and I want to update it with tags and some custom field, like 'rating' or 'mood' (not using any plugin, only WP built-in options for custom fields and REST API).
r = requests.post(WP_url, params = {'tags': tags, 'rating': rating}, auth = wp_auth)
Something like this. It works great for updating existing post parameters and fields, but I cannot find a way to create a custom field using Python API requests only.
I don't think it is possible to create a new field from request. Its depend on you WP Rest API Server, how it handles the excess argument you, if you API create a new field for any excess provided then only it will be possible to create new field.
How do I make a Python request for WordPress REST API to create a custom field?