Constraint Kinds

Every requirement Quell extracts from your specs is classified into a ConstraintKind. The kind determines how Quell generates and verifies the test.

Kinds

MUST_RAISE

A requirement that the function raises a specific exception under given conditions.

Extracted from: Raises: blocks in docstrings.

Example spec:

Raises:
    ValueError: If amount is zero or negative.

Generated test pattern:

def test_process_payment_must_raise_valueerror():
    import pytest
    with pytest.raises(ValueError):
        process_payment(request=PaymentRequest(amount=-1, currency="USD"))

Violation injection: Quell comments out the raise statement and confirms the test fails.

MUST_RETURN

A requirement about what the function returns.

Extracted from: Returns: blocks in docstrings.

Example spec:

Returns:
    dict with transaction_id, status, amount.

Violation injection: Quell replaces the return with return None and confirms the test fails.

BOUNDARY

A requirement that a value must be within a numeric range.

Extracted from: Pydantic Field(gt=0), Field(ge=18), Field(min_length=1); boundary phrases in docstrings ("must be greater than 0", "cannot exceed 500").

Example spec:

amount: float = Field(gt=0)

Generated test pattern:

def test_paymentrequest_amount_gt_boundary():
    import pytest
    with pytest.raises(Exception):
        PaymentRequest(amount=0, currency="USD")

Violation injection: Quell weakens the threshold to -9999 and confirms the test fails.

ENUM_VALID

A requirement that a value must be one of an allowed set.

Extracted from: Pydantic Literal["USD", "EUR", "GBP"] fields; enumeration phrases in docstrings ("one of USD, EUR, GBP").

Example spec:

currency: Literal["USD", "EUR", "GBP"]

Generated test pattern:

def test_paymentrequest_currency_invalid():
    import pytest
    with pytest.raises(Exception):
        PaymentRequest(amount=10.0, currency="JPY")

ENUM_INVALID

A requirement that a value must NOT be one of a set. Less common than ENUM_VALID.

NOT_NONE

A requirement that a return value or field must not be None.

BUG_REPRO

A requirement derived from a natural language bug description. Generated by quell reproduce.

Coverage: A BUG_REPRO requirement is never considered covered — it always generates a test. The test is written in a failing state (the bug exists) and passes once the bug is fixed.

MUTATION

A requirement derived from a mutation testing survivor (mutmut or Stryker). Used when enable_mutations = true.

CUSTOM

A requirement generated by the LLM for specs that don't fit the other categories.

Rule engine vs LLM

The LLM is never called for kinds the rule engine handles, unless the rule engine fails to produce a verified test after max_verification_attempts.