Round trip tested on the input file

Rather than on its notebook representation (#339)

HISTORY.rst

@@ -16,6 +16,7 @@ Release History
 
 - Fixed an inconsistent round trip (code cell with ``"cat"`` being converted to a markdown cell) in the ``py:light`` format (#339)
 - Commands like ``cat = x`` are not magic commands {#339)
+- ``jupytext --test`` now really compares the text (rather than the corresponding notebook) when run on text files (#339)
 
 
 1.2.4 (2019-09-19)

jupytext/cli.py

@@ -14,7 +14,7 @@
 from .header import recursive_update
 from .paired_paths import paired_paths, base_path, full_path, InconsistentPath
 from .combine import combine_inputs_with_outputs
-from .compare import test_round_trip_conversion, NotebookDifference
+from .compare import test_round_trip_conversion, NotebookDifference, compare
 from .kernels import kernelspec_from_language, find_kernel_specs, get_kernel_spec
 from .reraise import reraise
 from .version import __version__
@@ -351,11 +351,27 @@ def jupytext_single_file(nb_file, args, log):
     # a. Test round trip conversion
     if args.test or args.test_strict:
         try:
-            test_round_trip_conversion(notebook, dest_fmt,
-                                       update=args.update,
-                                       allow_expected_differences=not args.test_strict,
-                                       stop_on_first_error=args.stop_on_first_error)
-        except NotebookDifference as err:
+            # Round trip from an ipynb document
+            if fmt['extension'] == '.ipynb':
+                test_round_trip_conversion(notebook, dest_fmt,
+                                           update=args.update,
+                                           allow_expected_differences=not args.test_strict,
+                                           stop_on_first_error=args.stop_on_first_error)
+
+            # Round trip from a text file
+            else:
+                with open(nb_file) as fp:
+                    org_text = fp.read()
+
+                # If the destination is not ipynb, we convert to/back that format
+                if dest_fmt['extension'] != '.ipynb':
+                    dest_text = writes(notebook, fmt=dest_fmt)
+                    notebook = reads(dest_text, fmt=dest_fmt)
+
+                text = writes(notebook, fmt=fmt)
+                compare(org_text, text)
+
+        except (NotebookDifference, AssertionError) as err:
             sys.stdout.write('{}: {}'.format(nb_file, str(err)))
             return 1
         return 0

tests/test_cli.py

@@ -967,3 +967,18 @@ def test_339_ipynb(tmpdir, fmt):
     nbformat.write(nb, tmp_ipynb)
 
     assert jupytext([tmp_ipynb, '--to', fmt, '--test-strict']) == 0
+
+
+def test_339_py(tmpdir):
+    """Test that an incorrect round trip conversion on the text file is detected"""
+    tmp_py = str(tmpdir.join('test.py'))
+    with open(tmp_py, 'w') as fp:
+        fp.write("""# %%
+cat = 42
+""")
+
+    def erroneous_is_magic(line, language, comment_magics):
+        return 'cat' in line
+
+    with mock.patch('jupytext.magics.is_magic', erroneous_is_magic):
+        assert jupytext([tmp_py, '--to', 'ipynb', '--test-strict']) != 0