raise ValueError for unfitted idf vector

Catches an obscure AttributeError raised by calling .transform on a TfidfTransformer (with use_idf=True) that has not learned an idf vector.

Author: rlmv

sklearn/feature_extraction/text.py

@@ -915,6 +915,8 @@ def transform(self, X, copy=True):
             X.data += 1
 
         if self.use_idf:
+            if not hasattr(self, "_idf_diag"):
+                raise ValueError("idf vector not fitted")  
             expected_n_features = self._idf_diag.shape[0]
             if n_features != expected_n_features:
                 raise ValueError("Input has n_features=%d while the model"