Initial Conditions

For some problems, it is necessary for students to choose some configurations or initial conditions that can vary from student to student; handling all possible values of these initial conditions would require writing quite a few references. Instead, PyBryt offers the InitialCondition class to represent a value that will be set when the student’s submission is executed for use in writing annotations.

Writing a Reference with Initial Conditions

Using initial conditions in your references is pretty simple. Each InitialCondition has a name, the first argument to its constructor. This name is how the value in the student submission will be identified (but more on that later).

Once you’ve instantiated an InitialCondition, you can use it as normal in value and other types of annotations (except attribute annotations).

ic = pybryt.InitialCondition("foo")
pybryt.Value(ic)

Sometimes, however, you may want to look for a value derived from an initial condition. For this reason, InitialCondition supports all of Python’s arithmetic operators, and you can also apply transformations by writing functions:

pybryt.Value(ic + 2)
pybryt.Value(2 * ic - 3)

pybryt.Value(ic.apply(np.transpose).apply(np.linalg.norm))
pybryt.Value(ic.apply(lambda v: pow(v, 10, 73)))

Each statement inside the Value constructors above returns an InitialCondition. Initial conditions maintain a list of transformations that need to be applied to reach the final value for the annotation, and when a value is supplied from the submission, the transformations are applied in sequence to determine the value that the value annotations should look for.

Collecting Initial Conditions in Submissions

In order to link the initial conditions in the reference implementation to the values in the submission, you must call pybryt.set_initial_conditions in the submission. This function accepts a dictionary as its only argument mapping strings corresponding to the names of initial conditions to the values of those initial conditions. When PyBryt is not actively tracing, it has no effect; but, when called while PyBryt executes the submission, it tells PyBryt to store the initial conditions passed to it in the memory footprint for later use by the annotations.

For example, continuing the example from above, you would need to call

pybryt.set_initial_conditions({
    "foo": 2,
})

to set the initial condition.

This function can be called multiple times, but the memory footprint only retains a single store of initial conditions, so calling it multiple times with the same key will overwrite any old values of that key. (For example, calling it with {"foo": 1, "bar": 3} and then {"foo": 2} will result in initial conditions {"foo": 2, "bar": 3}).