Validators

didactic.api.validates decorates a method as a per-field check that runs at construction time:

import didactic.api as dx


class User(dx.Model):
    id: str
    email: str

    @dx.validates("email")
    def _email_must_have_at(self, value: str) -> bool:
        return "@" in value

The validator runs after the field's type translation. Its argument is the decoded value; its return value must be bool. False raises a ValidationError with a validates_failed entry.

Multiple validators per field

Multiple @validates methods on the same field run in declaration order. The first failure short-circuits the rest for that field.

Cross-field invariants

@validates runs against one field at a time. For a check that spans multiple fields, declare an axiom instead:

class Range(dx.Model):
    low: int
    high: int

    __axioms__ = [dx.axiom("low <= high")]

Validation error shape

try:
    User(id="u1", email="not-an-email")
except dx.ValidationError as exc:
    exc.entries        # tuple[ValidationErrorEntry, ...]
    exc.model          # type[Model]
    str(exc)           # rendered message

Each entry has:

• loc: a tuple of strings naming the location. For per-field validators this is (field_name,); for cross-field axioms it is ().
• type: a discriminator string (missing_required, extra_field, type_error, validates_failed, axiom_failed).
• msg: a human-readable description.

The set of type values is open; new versions may add more.