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Option to drop reference level in TreatmentContrast #263

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Option to drop reference level in TreatmentContrast #263

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9 changes: 5 additions & 4 deletions formulaic/transforms/contrasts.py
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Original file line number Diff line number Diff line change
Expand Up @@ -494,6 +494,7 @@ class TreatmentContrasts(Contrasts):
FACTOR_FORMAT_REDUCED = "{name}[T.{field}]"

base: Hashable = UNSET
drop: bool = False

@Contrasts.override
def _apply(
Expand All @@ -503,7 +504,7 @@ def _apply(
reduced_rank: bool = True,
sparse: bool = False,
) -> Union[pandas.DataFrame, numpy.ndarray, spsparse.spmatrix]:
if reduced_rank:
if reduced_rank or self.drop:
drop_index = self._find_base_index(levels)
mask = numpy.ones(len(levels), dtype=bool)
mask[drop_index] = False
Expand Down Expand Up @@ -536,7 +537,7 @@ def _get_coding_matrix(
matrix = spsparse.eye(n).tocsc()
else:
matrix = numpy.eye(n)
if reduced_rank:
if reduced_rank or self.drop:
drop_level = self._find_base_index(levels)
matrix = matrix[:, [i for i in range(matrix.shape[1]) if i != drop_level]]
return matrix
Expand All @@ -546,7 +547,7 @@ def get_coding_column_names(
self, levels: Sequence[Hashable], reduced_rank: bool = True
) -> Sequence[Hashable]:
base_index = self._find_base_index(levels)
if reduced_rank:
if reduced_rank or self.drop:
return [level for i, level in enumerate(levels) if i != base_index]
return levels

Expand All @@ -555,7 +556,7 @@ def get_coefficient_row_names(
self, levels: Sequence[Hashable], reduced_rank: bool = True
) -> Sequence[Hashable]:
base = levels[self._find_base_index(levels)]
if reduced_rank:
if reduced_rank or self.drop:
return [base, *(f"{level}-{base}" for level in levels if level != base)]
return levels

Expand Down