Glossary

Type descriptors

Modelity built-in mechanism for handling type parsing and value visiting logic.

It is provided by modelity.interface.ITypeDescriptor base class. Modelity evaluates type descriptors when model type is created, and later operates on closures created during model construction. Thanks to this approach the logic behind type support is quite simple, without degrading runtime performance too much.

Type descriptors can be built-in for the types Modelity supports, with a possibility to register custom types or 3rd party types if needed.

Data model

A subclass of modelity.model.Model class with data-specific fields and their types provided via type annotations like in dataclasses module.

Data parsing

First stage of data processing performed by Modelity when data model is either instantiated or modified. This stage, executed independently for each model field, guarantees that the right type was used for each field. If the value cannot be parsed to the right type, then modelity.exc.ParsingError is raised.

Model validation stage

Second stage of data processing, performed on demand by call to modelity.model.validate() function on an initialized model object.

Validation stage, as executed after data parsing stage, can assume that all fields are either unset, or set to a value of the right type. During validation, validators can access entire model to perform complex cross-field checks.

Unset field

Field that does not have value assigned.

Each unset field is set to the modelity.unset.Unset object, which is a singleton instance of the modelity.unset.UnsetType class. Following scenarios result in a field being unset:

• field is omitted in the model’s constructor and have no default value provided,

• field was deleted from a model, for example by calling del model.attr

• field was explicitly set to modelity.unset.Unset object.

Preprocessors

User-defined data parsing stage hooks for filtering input data before the data is passed further to the type parsers. Preprocessors can be defined for models by using modelity.model.field_preprocessor() decorator.

Preprocessors can be used, for example, to strip input data from white characters before that data is passed.

Since preprocessors are part of data parsing stage, each is executed independently for each matching field.

Postprocessors

User-defined data parsing stage hooks for adding custom field-level constraints or filters on values that were already converted to a right type. Postprocessors can be declared using modelity.model.field_postprocessor() decorator.