I'm a little horrified to suggest this seriously, but this could be a cool way to accomplish consistent field validation between server and client.

I wrote this little library https://github.com/fuhrysteve/marshmallow-jsonschema#complet... to convert marshmallow schemas to a JSON representation with the intention of passing it to a browser or mobile device for rendering. Imagine if you could also pass native code to be used by the client for validation. In other words: write your validator in python and use it server side - but also convert it to javascript so that the client can attempt to validate it before bothering to send a web request.