Fix bug when intercept is added after categorical variable (#102)

docs/CHANGELOG.md

@@ -6,6 +6,8 @@
 
 ### Maintenance and fixes
 
+- Fix bug when intercept is inserted after categorical variable (#102)
+
 ### Documentation
 
 ### Deprecation

formulae/contrasts.py

@@ -173,8 +173,6 @@ def pick_contrasts(group):
 
     used_subterms = set()
     codings = {}
-
     for name, components in group.items():
         codings[name] = ExpandedTerm(name, components).pick_contrast(used_subterms)
-
     return codings

formulae/terms/terms.py

@@ -1162,7 +1162,18 @@ def set_types(self, data, env):
 
     def _get_encoding_groups(self):
         components = {}
-        for term in self.common_terms:
+        # This is not the best fix, but we need the intercept to be in the first position
+        common_terms = self.common_terms.copy()
+        intercept_idx = -1
+        for i, term in enumerate(common_terms):
+            if isinstance(term, Intercept):
+                intercept_idx = i
+                break
+
+        if intercept_idx != -1:
+            common_terms.insert(0, common_terms.pop(intercept_idx))
+
+        for term in common_terms:
             if term.kind == "interaction":
                 components[term.name] = {c.name: c.kind for c in term.components}
             else:

tests/test_design_matrices.py

@@ -1217,3 +1217,24 @@ def f(*args, **kwargs):
 
     dm = design_matrices('f(x, y="abcd")', df)
     assert 'f(x, y="abcd")' in dm.common.terms
+
+
+def test_add_and_remove_intercept_works():
+    df = pd.DataFrame({"g": ["a", "a", "a", "b", "b"]})
+    dm = design_matrices("0 + g + 1", df)
+    assert np.allclose(
+        np.asarray(dm.common),
+        np.array([[0.0, 1.0], [0.0, 1.0], [0.0, 1.0], [1.0, 1.0], [1.0, 1.0]]),
+    )
+
+    dm = design_matrices("-1 + g + 1", df)
+    assert np.allclose(
+        np.asarray(dm.common),
+        np.array([[0.0, 1.0], [0.0, 1.0], [0.0, 1.0], [1.0, 1.0], [1.0, 1.0]]),
+    )
+
+    dm = design_matrices("0 + g + 0 + 1", df)
+    assert np.allclose(
+        np.asarray(dm.common),
+        np.array([[0.0, 1.0], [0.0, 1.0], [0.0, 1.0], [1.0, 1.0], [1.0, 1.0]]),
+    )

tests/test_eval_new_data.py

@@ -385,7 +385,7 @@ def test_new_group_specific_groups():
     group = design_matrices("1 + (1 + x | g1:g2)", df).group
     group_new = group.evaluate_new_data(df_2)
 
-    assert group_new.factors_with_new_levels == ("g1:g2", )
+    assert group_new.factors_with_new_levels == ("g1:g2",)
     assert np.array_equal(
         np.array(group_new),
         np.array([[0, 0, 0, 0, 1, 0, 0, 0, 0, 5], [0, 0, 0, 0, 1, 0, 0, 0, 0, 6]]),