Piping into black, flake8 or autopep8 made simpler

Standard input '-' argument is automatically added by Jupytext #154 #142

jupytext/cli.py

@@ -111,15 +111,15 @@ def parse_jupytext_args(args=None):
                         action='append',
                         help='Pipe the text representation of the notebook into another program, and read the'
                              'notebook back. For instance, reformat your notebook with:'
-                             "    jupytext notebook.ipynb --pipe 'black -'"
+                             "    jupytext notebook.ipynb --pipe black"
                              'If you want to reformat it and sync the paired representation, execute:'
-                             "    jupytext notebook.ipynb --sync --pipe 'black -'")
+                             "    jupytext notebook.ipynb --sync --pipe black")
     parser.add_argument('--exec',
                         action='append',
                         help='Pipe the text representation of the notebook into another program, and test that '
                              'the returned value is non zero. For instance, test that your notebook is pep8 compliant '
                              'with:'
-                             "    jupytext notebook.ipynb --exec 'flake8 -'")
+                             "    jupytext notebook.ipynb --exec flake8")
     parser.add_argument('--pipe-fmt',
                         default='auto:percent',
                         help='The format in which the notebook should be piped to other programs, when using the '
@@ -406,6 +406,9 @@ def load_paired_notebook(notebook, nb_file, log):
 def pipe_notebook(notebook, command, fmt='py:percent', update=True, preserve_outputs=True):
     """Pipe the notebook, in the desired representation, to the given command. Update the notebook
     with the returned content if desired."""
+    if command in ['black', 'flake8', 'autopep8']:
+        command = command + ' -'
+
     fmt = long_form_one_format(fmt, notebook.metadata)
     text = writes(notebook, fmt)
     process = subprocess.Popen(command.split(' '), stdout=subprocess.PIPE, stdin=subprocess.PIPE)
@@ -419,6 +422,10 @@ def pipe_notebook(notebook, command, fmt='py:percent', update=True, preserve_out
     if not update:
         return notebook
 
+    if not cmd_output:
+        sys.stderr.write("[jupytext] The command '{}' had no output. As a result, the notebook is empty. "
+                         "Is this expected? If not, use --exec rather than --pipe for this command.".format(command))
+
     piped_notebook = reads(cmd_output.decode('utf-8'), fmt)
 
     if preserve_outputs:

tests/test_black.py

@@ -54,7 +54,7 @@ def test_pipe_into_black():
     nb_org = new_notebook(cells=[new_code_cell('1        +1')])
     nb_dest = new_notebook(cells=[new_code_cell('1 + 1')])
 
-    nb_pipe = pipe_notebook(nb_org, 'black -')
+    nb_pipe = pipe_notebook(nb_org, 'black')
     compare(nb_dest, nb_pipe)
 
 
@@ -71,12 +71,12 @@ def test_pipe_into_autopep8():
 def test_pipe_into_flake8():
     # Notebook OK
     nb = new_notebook(cells=[new_code_cell('# correct code\n1 + 1')])
-    pipe_notebook(nb, 'flake8 -', update=False)
+    pipe_notebook(nb, 'flake8', update=False)
 
     # Notebook not OK
     nb = new_notebook(cells=[new_code_cell('incorrect code')])
     with pytest.raises(SystemExit):
-        pipe_notebook(nb, 'flake8 -', update=False)
+        pipe_notebook(nb, 'flake8', update=False)
 
 
 @requires_black
@@ -96,14 +96,14 @@ def test_apply_black_through_jupytext(tmpdir, nb_file):
 
     # Black in place
     writef(nb_org, tmp_ipynb)
-    jupytext([tmp_ipynb, '--pipe', 'black -'])
+    jupytext([tmp_ipynb, '--pipe', 'black'])
     nb_now = readf(tmp_ipynb)
     compare(nb_black, nb_now)
 
     # Write to another folder using dots
     script_fmt = os.path.join('..', 'script_folder//py:percent')
     writef(nb_org, tmp_ipynb)
-    jupytext([tmp_ipynb, '--to', script_fmt, '--pipe', 'black -'])
+    jupytext([tmp_ipynb, '--to', script_fmt, '--pipe', 'black'])
     assert os.path.isfile(tmp_py)
     nb_now = readf(tmp_py)
     nb_now.metadata = metadata
@@ -113,7 +113,7 @@ def test_apply_black_through_jupytext(tmpdir, nb_file):
     # Map to another folder based on file name
     writef(nb_org, tmp_ipynb)
     jupytext([tmp_ipynb, '--from', 'notebook_folder//ipynb', '--to', 'script_folder//py:percent',
-              '--pipe', 'black -', '--exec', 'flake8 -'])
+              '--pipe', 'black', '--exec', 'flake8'])
     assert os.path.isfile(tmp_py)
     nb_now = readf(tmp_py)
     nb_now.metadata = metadata