Derivations

A Derivation is identifies combinations of factor levels that select another level that is constructed with DerivedLevel. Derived levels for one factor must all have compatible derivations, which means that they must depend on the same factors in the same order, have the same window width, same window stride, and the same starting trial. However, ElseLevel is compatible with any derivation, as long as other levels in the same factor have compatible derivations.

class sweetpea.Derivation: Abstract class representing a derived-level specification.

class sweetpea.WithinTrial(predicate, factors)

Describes a level that is selected depending on levels from other factors, all within the same trial. The level is equivalent to one created via Window with a width of 1 and a stride of 1.

The factors that the level depends on can be non-derived or derived, but any derived factor in factors must not have a window stride greater than 1.

Parameters

predicate (Callable[[Any, ...], bool]) – a function that takes as many level names as factors in factors; the function should returns true if the combination of levels implies the result derivation
factors (List(Factor)) – a list of factors whose levels determine whether level with this derivation is selected

Return type

Derivation

class sweetpea.Transition(predicate, factors)

Describes a level that is selected depending on a combination of levels from other factors in the current trial and the immediately preceding trial. The level is equivalent to one created via Window with a width of 2 and a stride of 1.

The same as for WithinTrial, any derived factor in factors must not have a window stride greater than 1.

Parameters

predicate (Callable[[Dict[int, Any], ...], bool]) – a function that takes as many level dictionaries as factors in factors; in each dictionary, -1 is mapped to the level value for the previous trial, and 0 is mapped to the level value for the current trial; the function should return true if the combination of levels implies the result derivation
factors (List(Factor)) – a list of factors whose levels across trials determine whether a level with the returned derivation is selected

Return type

Derivation

class sweetpea.Window(predicate, factors, width, stride=1, start=None)

Creates a level that is selected depending on a combination of levels from other factors in the current trial and multiple preceding trials.

This class generalizes WithinTrial and Transition to select a level depending on width consecutive trials, and where trials that are assigned a level value are separated by stride-1 intervening trials. It also allows an initial start trial to override the default computation of a starting trial.

The same as for WithinTrial, any derived factor in factors must not have a window stride greater than 1.

Parameters

predicate (Callable[[Dict[int, Any], ...], bool]) – a function that takes as many level dictionaries as factors in factors; in each dictionary, 0 is mapped to the level value for the latest of width trials, and so on coutning backwards to 1-width; the function should return true if the combination of levels implies this level
factors (List(Factor)) – a list of factors whose levels across trials determine whether a level with the returned derivation is selected
width (int) – the number of trials of factors to consider when selecting the new, derived level
stride (int) – one more than the number of trials to skip between the trials that are considered when selecting the new, derived level
start (Optional[int]) – the first trail (counting from 0) that the derivation’s factor will have a level; a None value (the default) means that the first trial is automatically determined as the earliest where all factors in factors have a level for the trial and preceding width-1 trials; if start combined with width means that factors in factors will not have a level for some trials, then predicate must handle None values for the corresponding dictionaries and keys

Return type

Derivation