Move model before validators

[samuelcolvin]

As discussed yesterday with @dmontagu.

I know this changes the behaviour of test_model_validator_before, but I think it's actually more correct.

Previously before model validators were called when the input is a model, even when validation wasn't being run. Now before validators are called in the same scenarios as the rest of model validation is run:

• on both instances of the model type and instances of subclasses if revalidate_instances='always'
• only on instances of subclasses if revalidate_instances='subclass-instances'
• on neither instances of the model type nor instances of subclasses if revalidate_instances='never'

As previously, before model validators are always called when the input does not pass isinstance(input, Model).

This change does mean the logic for when before validators are called differs from wrap and after model validators - but honestly I think wrap and after model validators are wrong, although I'm not sure that's worth changing.

We should probably add tests for:

• model_validator(mode='before') on RootModel
• model_validator(mode='before') on dataclass

Selected Reviewer: @adriangb

[cloudflare-pages]

Deploying with    Cloudflare Pages

Latest commit:	652ed99
Status:	✅  Deploy successful!
Preview URL:	https://92032b70.pydantic-docs2.pages.dev
Branch Preview URL:	https://move-model-before-validators.pydantic-docs2.pages.dev

View logs

[adriangb]

What's the change of behavior for test_model_validator_before? I don't see that test being changed.

[samuelcolvin]

please review.

[samuelcolvin]

Merging this, happy to change/revert/add more if needed.