Added flag for unknown value error

rdt/transformers/categorical.py

@@ -218,11 +218,20 @@ class OneHotEncodingTransformer(BaseTransformer):
     is found and 0s on the rest.
 
     Null values are considered just another category.
+
+    Args:
+        error_on_unknown (bool):
+
+            If a value that was not seen during the fit stage is passed to
+            transform, then an error will be raised if this is True.
     """
 
     dummy_na = None
     dummies = None
 
+    def __init__(self, error_on_unknown=True):
+        self.error_on_unknown = error_on_unknown
+
     def fit(self, data):
         """Fit the transformer to the data.
 
@@ -248,7 +257,7 @@ def transform(self, data):
         dummies = pd.get_dummies(data, dummy_na=self.dummy_na)
         array = dummies.reindex(columns=self.dummies, fill_value=0).values.astype(int)
         for i, row in enumerate(array):
-            if np.all(row == 0):
+            if np.all(row == 0) and self.error_on_unknown:
                 raise ValueError(f"The value {data[i]} was not seen during the fit stage.")
 
         return array

tests/transformers/test_boolean.py

@@ -180,5 +180,5 @@ def test_reverse_transform_not_null_values(self):
         # Asserts
         expected = np.array([True, False, True])
 
-        assert type(result) == pd.Series
+        assert isinstance(result, pd.Series)
         np.testing.assert_equal(result.values, expected)