add LF number check (#1428)

Author: paroma

snorkel/labeling/model/label_model.py

@@ -547,6 +547,8 @@ def _set_class_balance(
 
     def _set_constants(self, L: np.ndarray) -> None:
         self.n, self.m = L.shape
+        if self.m < 3:
+            raise ValueError(f"L_train should have at least 3 labeling functions")
         self.t = 1
 
     def _create_tree(self) -> None:

test/labeling/model/test_label_model.py

@@ -38,6 +38,10 @@ def test_L_form(self):
         with self.assertRaisesRegex(ValueError, "L_train has cardinality"):
             label_model.fit(L, n_epochs=1)
 
+        L = np.array([[0], [1], [-1]])
+        with self.assertRaisesRegex(ValueError, "L_train should have at least 3"):
+            label_model.fit(L, n_epochs=1)
+
     def test_class_balance(self):
         label_model = LabelModel(cardinality=2, verbose=False)
         # Test class balance