In dry run mode, only print output if there would have been a change

Closes #152

nbstripout/_nbstripout.py

@@ -332,45 +332,44 @@ def status(git_config, install_location=INSTALL_LOCATION_LOCAL, verbose=False):
         return 1
 
 def process_jupyter_notebook(input_stream, output_stream, args, extra_keys, filename='input from stdin'):
-    any_change = False
-
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", category=UserWarning)
         nb = nbformat.read(input_stream, as_version=nbformat.NO_CONVERT)
 
     nb_orig = copy.deepcopy(nb)
-    nb = strip_output(nb, args.keep_output, args.keep_count, args.keep_id,
-                        extra_keys, args.drop_empty_cells,
-                        args.drop_tagged_cells.split(), args.strip_init_cells,
-                        _parse_size(args.max_size))
+    nb_stripped = strip_output(nb, args.keep_output, args.keep_count,
+                               args.keep_id, extra_keys, args.drop_empty_cells,
+                               args.drop_tagged_cells.split(),
+                               args.strip_init_cells, _parse_size(args.max_size))
 
-    if nb_orig != nb:
-        any_change = True
+    any_change = nb_orig != nb_stripped
 
     if args.dry_run:
-        output_stream.write(f'Dry run: would have stripped {filename}\n')
-    else:
-        if output_stream.seekable():
-            output_stream.seek(0)
-            output_stream.truncate()
-        with warnings.catch_warnings():
-            warnings.simplefilter("ignore", category=UserWarning)
-            nbformat.write(nb, output_stream)
-        output_stream.flush()
+        if any_change:
+            output_stream.write(f'Dry run: would have stripped {filename}\n')
+        return any_change
+
+    if output_stream.seekable():
+        output_stream.seek(0)
+        output_stream.truncate()
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", category=UserWarning)
+        nbformat.write(nb_stripped, output_stream)
+    output_stream.flush()
     return any_change
 
 def process_zeppelin_notebook(input_stream, output_stream, args, extra_keys, filename='input from stdin'):
-    any_change = False
-
     nb = json.load(input_stream, object_pairs_hook=collections.OrderedDict)
     nb_orig = copy.deepcopy(nb)
     nb_stripped = strip_zeppelin_output(nb)
-    if nb_orig != nb_stripped:
-        any_change = True
+
+    any_change = nb_orig != nb_stripped
 
     if args.dry_run:
-        output_stream.write(f'Dry run: would have stripped {filename}\n')
+        if any_change:
+            output_stream.write(f'Dry run: would have stripped {filename}\n')
         return any_change
+
     if output_stream.seekable():
         output_stream.seek(0)
         output_stream.truncate()

tests/e2e_notebooks/test_nochange.ipynb

@@ -0,0 +1,61 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This notebook should not be changed by nbstripout."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "1+1 # This cell has `\"init_cell:\" true`"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "metadata": {
+    "keep_output": true
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "4"
+      ]
+     },
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "2+2 # This cell has `\"keep_output:\" true`"
+   ]
+  }
+ ],
+ "metadata": {
+  "celltoolbar": "Edit Metadata",
+  "kernelspec": {
+   "display_name": "Python 2",
+   "language": "python",
+   "name": "python2"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 2
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.11"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}

tests/test_end_to_end.py

@@ -37,8 +37,9 @@
 ]
 
 DRY_RUN_CASES = [
-    ("test_metadata.ipynb", []),
-    ("test_zeppelin.zpln", ["--mode", "zeppelin"]),
+    ("test_metadata.ipynb", [], True),
+    ("test_zeppelin.zpln", ["--mode", "zeppelin"], True),
+    ("test_nochange.ipynb", [], False),
 ]
 
 ERR_OUTPUT_CASES = [
@@ -110,9 +111,9 @@ def test_end_to_end_file(input_file: str, expected_file: str, args: List[str], t
         assert not pc.stdout and p.read_text() == expected
 
 
-@pytest.mark.parametrize("input_file, extra_args", DRY_RUN_CASES)
+@pytest.mark.parametrize("input_file, extra_args, any_change", DRY_RUN_CASES)
 @pytest.mark.parametrize("verify", (True, False))
-def test_dry_run_stdin(input_file: str, extra_args: List[str], verify: bool):
+def test_dry_run_stdin(input_file: str, extra_args: List[str], any_change:bool, verify: bool):
     expected = "Dry run: would have stripped input from stdin\n"
 
     with open(NOTEBOOKS_FOLDER / input_file, mode="r") as f:
@@ -122,23 +123,22 @@ def test_dry_run_stdin(input_file: str, extra_args: List[str], verify: bool):
         pc = run(args, stdin=f, stdout=PIPE, universal_newlines=True)
         output = pc.stdout
 
-    assert output == expected
-    assert pc.returncode == (1 if verify else 0)
+    assert output == (expected if any_change else '')
+    assert pc.returncode == (1 if verify and any_change else 0)
 
 
-@pytest.mark.parametrize("input_file, extra_args", DRY_RUN_CASES)
+@pytest.mark.parametrize("input_file, extra_args, any_change", DRY_RUN_CASES)
 @pytest.mark.parametrize("verify", (True, False))
-def test_dry_run_args(input_file: str, extra_args: List[str], verify: bool):
+def test_dry_run_args(input_file: str, extra_args: List[str], any_change: bool, verify: bool):
     expected_regex = re.compile(f"Dry run: would have stripped .*[/\\\\]{input_file}\n")
     args = [nbstripout_exe(), str(NOTEBOOKS_FOLDER / input_file), "--dry-run", ] + extra_args
     if verify:
         args.append("--verify")
     pc = run(args, stdout=PIPE, universal_newlines=True)
     output = pc.stdout
 
-    assert expected_regex.match(output)
-    if verify:
-        assert pc.returncode == 1
+    assert expected_regex.match(output) if any_change else output == ''
+    assert pc.returncode == (1 if verify and any_change else 0)
 
 
 @pytest.mark.parametrize("input_file, expected_errs, extra_args", ERR_OUTPUT_CASES)