Custom Types: Value Substitution

This article is a sequel to the previous article and focuses on implementing value substitution for custom types.

Step 1. Enhance Definitions

First, enhance the NumericProps class by adding a value property. This property will hold the value of the numeric type.

from niltype import Nilable
from d42.custom_type import Props

class NumericProps(Props):
    @property
    def value(self) -> Nilable[str]:
        return self.get("value")

Then, implement the __call__ method in NumericSchema. This method enables declaring a schema with a value, like schema_numeric("42").

from typing import Self
from d42.custom_type import CustomSchema, Props

class NumericSchema(CustomSchema[NumericProps]):
    def __call__(self, value: str) -> Self:
        return self.__class__(self.props.update(value=value))

Step 2. Refine Type Representation

Modify the __represent__ method in NumericSchema to include the numeric value in its representation.

from niltype import Nil
from d42.custom_type.visitors import Representor

class NumericSchema(CustomSchema[NumericProps]):
    ...

    def __represent__(self, visitor: Representor, indent: int = 0) -> str:
        if self.props.value is not Nil:
            return f"{visitor.name}.numeric({self.props.value!r})"
        return f"{visitor.name}.numeric"

print(schema_numeric("1234"))
# Output:
# schema.numeric('1234')

Step 3. Fine-Tune Type Generation

Adjust the __generate__ method in NumericSchema to account for a predefined value. If a value is already specified, it returns this value instead of generating a new one.

from random import randint
from niltype import Nil
from d42.custom_type.visitors import Generator

class NumericSchema(CustomSchema[NumericProps]):
    ...

    def __generate__(self, visitor: Generator) -> str:
        if self.props.value is not Nil:
            return self.props.value
        number = randint(0, 2147483647)  # int32 max
        return str(number)

from d42 import fake
sch = schema_numeric("1234")
print(fake(sch))

# Output:
# 1234

Step 4. Implement Value Validation

Enhance the __validate__ method to include an additional check. If a specific value is set in the schema's properties, it validates that the incoming value matches this set value.

from typing import Any
from niltype import Nil
from d42.custom_type.visitors import Validator
from d42.custom_type import PathHolder, ValidationResult
from d42.custom_type.errors import TypeValidationError, ValueValidationError

class NumericSchema(CustomSchema[NumericProps]):
    def __validate__(self, visitor: Validator, value: Any, path: PathHolder) -> ValidationResult:
        result = visitor.make_validation_result()

        if not (isinstance(value, str) and all(char in "1234567890" for char in value)):
            result.add_error(TypeValidationError(path, value, "numeric str"))

        if (self.props.value is not Nil) and (value != self.props.value):
            result.add_error(ValueValidationError(path, value, self.props.value))

        return result

from d42 import validate_or_fail
sch = schema_numeric("1234")

print(validate_or_fail(sch, "1234"))
# Output:
# True

print(validate_or_fail(sch, "5678"))
# Output:
# d42.ValidationException:
# - Value <class 'str'> must be equal to '1234', but '5678' given

Step 5. Add Value Substitution

The __substitute__ method is used for value substitution in custom types. It first validates the incoming value and, if valid, updates the properties with this new value.

Unlike other features such as representation, generation, and validation, which can be implemented independently and as needed, value substitution based on validation. So first need to validate incoming value and then update props.

from typing import Any, Self
from d42.custom_type.visitors import Substitutor
from d42.custom_type.utils import make_substitution_error

class NumericSchema(CustomSchema[NumericProps]):
    ...

    def __substitute__(self, visitor: Substitutor, value: Any) -> Self:
        result = visitor.validator.visit(self, value=value)
        if result.has_errors():
            raise make_substitution_error(result, visitor.formatter)

        return self.__class__(self.props.update(value=value))

print(schema_numeric % "1234")
# Output
# schema.numeric("1234")

print(schema_numeric("1234") % "5678")
# Output:
# d42.substitution.errors.SubstitutionError:
# - Value <class 'str'> must be equal to '1234', but '5678' given

tip

For the complete code snippet of this implementation, please refer to this page