Merge pull request fastai#174 from radekosmulski/make_it_clear_that_t…

…ransforms_are_needed

Add assertion

fastai/dataset.py

@@ -365,6 +365,8 @@ def from_paths(cls, path, bs=64, tfms=(None,None), trn_name='train', val_name='v
         Returns:
             ImageClassifierData
         """
+        assert isinstance(tfms[0], Transforms) and isinstance(tfms[1], Transforms), \
+            "please provide transformations for your train and validation sets"
         trn,val = [folder_source(path, o) for o in (trn_name, val_name)]
         test_fnames = read_dir(path, test_name) if test_name else None
         datasets = cls.get_ds(FilesIndexArrayDataset, trn, val, tfms, path=path, test=test_fnames)