raise StringLengthException if vectoriser is applied to strings that …

…are not all greater in length than ngram_size

string_grouper/string_grouper.py

@@ -194,6 +194,11 @@ class StringGrouperNotFitException(Exception):
     pass
 
 
+class StringLengthException(Exception):
+    """Raised when vectoriser is fit on strings that are not of length greater than ngram size"""
+    pass
+
+
 class StringGrouper(object):
     def __init__(self, master: pd.Series,
                  duplicates: Optional[pd.Series] = None,
@@ -258,6 +263,13 @@ def n_grams(self, string: str) -> List[str]:
 
     def fit(self) -> 'StringGrouper':
         """Builds the _matches list which contains string matches indices and similarity"""
+
+        # Validate match strings length
+        if not StringGrouper._strings_are_of_sufficient_length(self._master,  self._config.ngram_size) or \
+                (self._duplicates is not None
+                 and not StringGrouper._strings_are_of_sufficient_length(self._duplicates, self._config.ngram_size)):
+            raise StringLengthException('Input string lengths are not all greater than n_gram length')
+
         master_matrix, duplicate_matrix = self._get_tf_idf_matrices()
 
         # Calculate the matches using the cosine similarity
@@ -697,6 +709,16 @@ def _is_series_of_strings(series_to_test: pd.Series) -> bool:
             return False
         return True
 
+    @staticmethod
+    def _strings_are_of_sufficient_length(series_to_test: pd.Series, ngram_size: int) -> bool:
+        if not isinstance(series_to_test, pd.Series):
+            return False
+        elif series_to_test.to_frame().applymap(
+                    lambda x: not len(x) >= ngram_size
+                ).squeeze(axis=1).all():
+            return False
+        return True
+
     @staticmethod
     def _is_input_data_combination_valid(duplicates, master_id, duplicates_id) -> bool:
         if duplicates is None and (duplicates_id is not None) \

string_grouper/test/test_string_grouper.py

@@ -6,7 +6,7 @@
     DEFAULT_REGEX, DEFAULT_NGRAM_SIZE, DEFAULT_N_PROCESSES, DEFAULT_IGNORE_CASE, \
     StringGrouperConfig, StringGrouper, StringGrouperNotFitException, \
     match_most_similar, group_similar_strings, match_strings, \
-    compute_pairwise_similarities
+    compute_pairwise_similarities, StringLengthException
 from unittest.mock import patch
 
 
@@ -822,6 +822,11 @@ def test_prior_matches_added(self):
         # All strings should now match to the same "master" string
         self.assertEqual(1, len(df.deduped.unique()))
 
+    def test_group_similar_strings_stopwords(self):
+        """StringGrouper shouldn't raise a ValueError if all strings are shorter than 3 characters"""
+        with self.assertRaises(StringLengthException):
+            StringGrouper(pd.Series(['zz', 'yy', 'xx'])).fit()
+
 
 if __name__ == '__main__':
     unittest.main()