Refine Python pattern boundaries and Pydantic guidance

patterns/data-models.md

@@ -11,16 +11,7 @@ A lot of messy Python comes from making one class do four jobs at once:
 - serialize output
 - own persistence or business behavior
 
-Mature codebases usually split those concerns.
-
-The recurring pattern is:
-
-- use explicit boundary models for validated input and output
-- keep serialization explicit
-- use lightweight value-like classes for simple internal state
-- avoid turning every data container into a god object
-
-## The pattern
+Mature codebases usually split those concerns:
 
 1. Use dedicated boundary models where data enters or leaves the system.
 2. Validate and serialize explicitly.
@@ -43,9 +34,7 @@ Use small value-like objects when:
 
 ## When not to use
 
-Do not make every internal object a heavyweight validation model.
-
-Do not scatter custom `to_dict()` logic across the codebase.
+Do not treat a boundary model as proof that every internal object should also become a heavyweight validation model.
 
 Do not let one model become schema, ORM wrapper, validator, service object, and side-effect manager at the same time.
 
@@ -117,9 +106,9 @@ That is not a model. That is a junk drawer.
 
 - `pydantic/main.py:253-264` says `BaseModel.__init__` parses and validates input data and raises `ValidationError` on bad input.
 - `pydantic/main.py:455-519` defines `model_dump(...)` as an explicit serialization step with caller-controlled include/exclude behavior.
-- `pydantic/main.py:721-768` defines `model_validate(...) -> Self` as a named boundary-crossing API.
-- `docs/index.md:82-107` pairs model creation with `model_dump()` instead of treating instances as already wire-ready.
-- `docs/index.md:109-124` shows invalid external input failing loudly with `ValidationError`.
+- `pydantic/main.py:721-768` defines `model_validate(...) -> Self` as a named boundary-crossing API with explicit validation options.
+- `docs/index.md:68-107` shows raw external data being coerced into typed fields and then dumped back out explicitly.
+- `docs/index.md:109-152` shows invalid external input failing with structured per-field validation errors instead of ad hoc strings.
 
 ### Attrs
 
@@ -131,6 +120,10 @@ That is not a model. That is a junk drawer.
 
 - `src/_pytest/timing.py:24-64` models `Instant` and `Duration` as frozen dataclasses for simple internal timing state.
 
+## Related comparison
+
+- `comparison/pydantic-vs-python.md`
+
 ## Bottom line
 
 Boundary models should validate and serialize cleanly.