ERR Clarify error message (#81)

* better error message if a notebook/directory isn't passed

* better error message if a notebook/directory isn't passed

README.rst

@@ -73,7 +73,7 @@ Format your notebooks using :code:`black`:
 
 .. code-block:: bash
 
-    $ nbqa black .
+    $ nbqa black . --line-length=96
     reformatted tweet-sentiment-roberta-pytorch.ipynb
     All done! ✨ 🍰 ✨
     1 files reformatted.
@@ -144,14 +144,23 @@ could add to your :code:`.pre-commit-config.yaml` file:
     hooks:
       - id: nbqa
         args: ['flake8']
+        name: nbqa-flake8
   - repo: https://github.com/MarcoGorelli/nbQA-mirror-1
     rev: master
     hooks:
       - id: nbqa
         args: ['isort']
+        name: nbqa-isort
+  - repo: https://github.com/MarcoGorelli/nbQA-mirror-2
+    rev: master
+    hooks:
+      - id: nbqa
+        args: ['mypy']
+        name: nbqa-mypy
 
-Note that, because the repository keys need to be unique, you will need to use a
-different mirror repository for each pre-commit hook you wish to use.
+It's entirely optional which tool to use with which mirror, the purpose of having
+these mirror is to avoid duplicate keys in the :code:`.pre-commit.yaml` file, but
+they're all the same.
 
 See Also
 --------

nbqa/__main__.py

@@ -23,12 +23,22 @@ def _parse_args(raw_args):
     """
     parser = argparse.ArgumentParser(
         description="Adapter to run any code-quality tool on a Jupyter notebook.",
-        usage="nbqa <command> <notebook or directory> <flags>",
+        usage=(
+            "nbqa <command> <notebook or directory> <flags>\n"
+            "e.g. `nbqa flake8 my_notebook.ipynb --ignore=E203`"
+        ),
     )
     parser.add_argument("command", help="Command to run, e.g. `flake8`.")
     parser.add_argument("root_dir", help="Notebook or directory to run command on.")
     parser.add_argument("--version", action="version", version=f"nbQA {__version__}")
-    args, kwargs = parser.parse_known_args(raw_args)
+    try:
+        args, kwargs = parser.parse_known_args(raw_args)
+    except SystemExit as e:
+        msg = (
+            "Please specify both a command and a notebook/directory, e.g.\n"
+            "nbqa flake8 my_notebook.ipynb"
+        )
+        raise ValueError(msg) from e
     command = args.command
     root_dir = args.root_dir
     return command, root_dir, kwargs

tests/test_missing_root_dir.py

@@ -0,0 +1,12 @@
+import pytest
+
+from nbqa.__main__ import main
+
+
+def test_missing_root_dir():
+    msg = (
+        "Please specify both a command and a notebook/directory, e.g.\n"
+        "nbqa flake8 my_notebook.ipynb"
+    )
+    with pytest.raises(ValueError, match=msg):
+        main(["flake8", "--ignore=E203"])