Predicates

In the world of validation, predicates are simply expressions that return a True or False for a given condition. In Python type hints, predicates can be expressed like this:

A = TypeVar('A')

PredicateFunc = Callable[[A], bool]

Koda Validate has a Predicate class based on this concept. In Koda Validate, Predicates are used to enrich Validators by performing additional validation after the data has been verified to be of a specific type or shape.

from koda_validate import IntValidator, Min

int_validator = IntValidator(Min(5))  # `Min` is a `Predicate`

Usage:

>>> int_validator(6)
Valid(val=6)

>>> int_validator("a string")
Invalid(err_type=TypeErr(expected_type=<class 'int'>), ...)

>>> int_validator(4)
Invalid(err_type=PredicateErrs(predicates=[Min(minimum=5, exclusive_minimum=False)]), ...)

As you can see the value 4 passes the int type check but fails to pass the Min(5) predicate.

Because we know that predicates don’t change the type or value of their inputs, we can sequence an arbitrary number of Predicates together, and validate them all.

from koda_validate import (IntValidator, Min, Max, MultipleOf, Invalid,
                           Valid, PredicateErrs)

int_validator = IntValidator(Min(5), Max(20), MultipleOf(4))

assert int_validator(12) == Valid(12)

invalid_result = int_validator(23)
assert isinstance(invalid_result, Invalid)
assert invalid_result.err_type == PredicateErrs([Max(20), MultipleOf(4)])

Here int_validator has 3 Predicates, but we could have as many as we want. Note that failing Predicates are returned within a PredicateErrs instance. We are only able to return all the failing Predicates because we know that each Predicate will not be able to change the value.

Predicates are easy to write – take a look at Extension for more details.