02.04.03 — .otherwise(...) and _any_of#
How Ocarina expresses a logical OR between two predicates without breaking error aggregation.
The problem#
Concrete case: you want “age == 18 OR age <= 65.” The ROP/invariants syntax is a strict chain of assertions, and every .assert_that() is an AND. How do you express an OR?
Code#
def otherwise(self, fallback: Predicate[T], *, msg: str | None = None) -> ValidationAssertBlock[T]:
if self._last_predicate is None:
raise RuntimeError("otherwise() must follow assert_that().")
fallback_with_msg = _with_msg(fallback, msg, self._name)
combined = _any_of(
self._last_predicate, *([*self._otherwise_predicates, fallback_with_msg])
)
self._chain._steps.pop() # replace the last step
self._chain.add_assertion(self._value, combined, self._name)
self._last_predicate = combined
self._otherwise_predicates.append(fallback_with_msg)
return self
- Guard:
otherwise() has to follow an assert_that(). With no prior predicate it raises. (In practice the type-checker already rules it out — ValidationStartBlock.otherwise doesn’t exist.) - Combination: combine previous predicate + all accumulated
otherwise predicates + the new one, via _any_of. Result: a composite predicate. - Step replacement: pop the last step from
_ValidationChain and push the combined one. Otherwise the original predicate AND the combined one would both be present, and the original error would resurface.
_any_of#
def _any_of[T](*predicates: _PredicateWithMsg[T]) -> _PredicateWithMsg[T]:
def _try_predicate(p: _PredicateWithMsg[T], value: T) -> InvariantViolationError | None:
try:
p(value)
except InvariantViolationError as exc:
return exc
else:
return None
def combined(value: T) -> None:
errors = []
for p in predicates:
error = _try_predicate(p, value)
if error is None:
return # ✅ one predicate passes → all pass
errors.append(error)
formatted_error_messages = " | " + "\n | ".join(str(e) for e in errors)
msg = (
f"All predicates failed for value {value!r}.\n"
"» At least one of the following conditions must be satisfied:\n"
f"{formatted_error_messages}"
)
raise InvariantViolationError(msg)
return _PredicateWithMsg(combined)
- First success wins: the moment one predicate passes,
combined returns None. - All fail: build an aggregated message listing every failure, separated by
|. - Result is a
_PredicateWithMsg: plug it back into the chain like any other predicate. - Aggregation is recursive: 3 successive
otherwise() calls fold (P_orig OR P_otherwise1 OR P_otherwise2 OR P_otherwise3) into one predicate.
Example#
validate(age, name="age")
.assert_that(is_equal_to(18), msg="Must be 18 or at most 65")
.otherwise(is_less_than_or_equal_to(65), msg="Must be 18 or at most 65")
.execute()
.raise_if_invalid()
- If
age == 18: is_equal_to(18) passes → OK. - If
age == 30: is_equal_to(18) raises, is_less_than_or_equal_to(65) passes → OK. - If
age == 70: both raise → InvariantViolationError with:
age: All predicates failed for value 70.
» At least one of the following conditions must be satisfied:
| age: Must be 18 or at most 65
| age: Must be 18 or at most 65
(The duplicated message comes from passing the same msg= twice. Intentional here — we want a single statement of intent.)
