Improve error message when only --test is passed

#339

jupytext/cli.py

@@ -225,12 +225,15 @@ def log(text):
         print_paired_paths(args.notebooks[0], args.input_format)
         return 1
 
+    if (args.test or args.test_strict) and not args.to and not args.output and not args.sync:
+        raise ValueError('Please provide one of --to, --output or --sync')
+
     if not args.to and not args.output and not args.sync \
             and not args.pipe and not args.check \
-            and not args.test and not args.test_strict \
             and not args.update_metadata and not args.set_kernel \
             and not args.execute:
-        raise ValueError('Please select an action')
+        raise ValueError('Please provide one of --to, --output, --sync, --pipe, '
+                         '--check, --update_metadata, --set-kernel or --execute')
 
     if args.output and len(args.notebooks) != 1:
         raise ValueError('Please input a single notebook when using --output')

tests/test_cli.py

@@ -982,3 +982,11 @@ def erroneous_is_magic(line, language, comment_magics):
 
     with mock.patch('jupytext.magics.is_magic', erroneous_is_magic):
         assert jupytext([tmp_py, '--to', 'ipynb', '--test-strict']) != 0
+
+
+def test_339_require_to(tmpdir):
+    """Test that the to argument is asked for when a `--test` command is provided"""
+    tmp_py = str(tmpdir.join('test.py'))
+
+    with pytest.raises(ValueError, match='--to'):
+        jupytext([tmp_py, '--test-strict'])