Invariants

Invariants provide logic for determining whether a value satisfies a value annotation. They can be used to ensure that annotations can still be satisfied independent of the different ways students format their answers.

Invariant Structure

Invariants are subclasses of the abstract base class invariants.invariant. All subclasses implement the static run method, which takes in a list of objects and returns a transformed list of objects that contains all versions of every object in the input list which are considered equal.

Consider an invariant for string capitalization. The run method of this invariant takes in a list of objects and returns the same list but with every string lowercased. For a matrix transposition invariant, the run method might return a list with every 2D array’s transpose included, as well as the original array.

Invariants have a custom __new__ method that calls the run method, so that they function as callables rather than classes.

PyBryt supports custom invariants. To create your own invariant, subclass invariants.invariant and implement the run method.

Built-In Invariants

A list of built-in invariants can be found here.