TypeAdapter and discriminated unions

Not everything you validate is a model. A database query returns a list of rows, a queue delivers a dictionary keyed by id, a streaming endpoint emits lines that are one of several shapes. TypeAdapter gives any type hint the same validate_python, validate_json, dump_json, and json_schema methods a model has, so you do not need a wrapper class whose only field is items.

The second idea in this lesson is what to do when a value can be one of several models. A plain union makes Pydantic try each member in turn and pick the best fit, which is slow and produces vague errors. A discriminated union names one field, usually type, that says which model applies, so validation is a single dictionary lookup and an error points at the right model.