Validate a manager with field and non-field rule errors

This recipe exercises the public Rule API directly and is suitable for a unit test. Defining named functions keeps their source available to inspect.

from dataclasses import dataclass

from django.core.exceptions import NON_FIELD_ERRORS

from general_manager.rule import Rule


@dataclass
class Project:
    name: str | None


@dataclass
class Booking:
    starts_at: int
    ends_at: int
    project: Project | None = None


def ordered(booking: Booking) -> bool:
    return booking.starts_at < booking.ends_at


field_rule = Rule(
    ordered,
    custom_error_message="Start {starts_at} must be before the end.",
)

assert field_rule.evaluate(Booking(starts_at=20, ends_at=10)) is False
assert field_rule.get_error_message() == {
    "starts_at": "Start 20 must be before the end.",
    "ends_at": "Start 20 must be before the end.",
}


def maintenance_window_closed(_booking: Booking) -> bool:
    return False


non_field_rule = Rule(
    maintenance_window_closed,
    custom_error_message="Bookings are temporarily disabled.",
    ignore_if_none=False,
)

assert non_field_rule.evaluate(Booking(starts_at=1, ends_at=2)) is False
assert non_field_rule.get_error_message() == {
    NON_FIELD_ERRORS: "Bookings are temporarily disabled."
}


def project_selected(booking: Booking) -> bool:
    return booking.project is not None


dotted_rule = Rule(
    project_selected,
    custom_error_message="A project is required; received {project.name}.",
    ignore_if_none=False,
)

assert dotted_rule.evaluate(Booking(starts_at=1, ends_at=2, project=None)) is False
assert dotted_rule.get_error_message() == {
    "project": "A project is required; received None."
}

The first custom template intentionally covers only starts_at; placeholders are optional and do not change the existing starts_at and ends_at error keys. The second message is static. The last rule demonstrates a dotted attribute path without calling a method. With ignore_if_none=False, an intermediate or final None renders as "None" when the failed rule is formatted.

Only dot-separated Python identifiers are supported in placeholders. Syntax and predicate roots are validated at rule construction. Paths are validated against declared manager schemas during shared manager startup for normal application managers; a manager declared later validates on its first public use after app readiness, before CRUD or field evaluation. Invalid syntax, unrelated roots, and unknown or non-traversable paths raise InvalidErrorTemplateError. Calls, indexes, conversions, format specifications, filters, arbitrary expressions, and literal-brace escaping are unsupported. Passing and skipped rules return no message.

MissingErrorTemplateVariableError remains available from general_manager.rule.rule for compatibility, but a template may omit predicate variables without raising it.

The guaranteed non-empty fallback was added in 0.62.2. Earlier versions could return None after a failed predicate when no AST handler produced a message.

See the concept page, task guide, and API reference.