add a test of arg parsing

dp_creator_ii/tests/test_arg_parser.py

@@ -0,0 +1,38 @@
+from pathlib import Path
+from argparse import ArgumentTypeError
+
+import pytest
+
+from dp_creator_ii import get_arg_parser, existing_csv
+
+
+def test_help():
+    help = (
+        get_arg_parser()
+        .format_help()
+        # argparse doesn't actually know the name of the script
+        # and inserts the name of the running program instead.
+        .replace("__main__.py", "dp-creator-ii")
+        .replace("pytest", "dp-creator-ii")
+        # Text is different under Python 3.9:
+        .replace("optional arguments:", "options:")
+    )
+    print(help)
+
+    readme_md = (Path(__file__).parent.parent.parent / "README.md").read_text()
+    assert help in readme_md
+
+
+def test_arg_validation_no_file():
+    with pytest.raises(ArgumentTypeError, match="No such file: no-such-file"):
+        existing_csv("no-such-file")
+
+
+def test_arg_validation_not_csv():
+    with pytest.raises(ArgumentTypeError, match='Must have ".csv" extension:'):
+        existing_csv(Path(__file__).parent / "fixtures" / "fake.ipynb")
+
+
+def test_arg_validation_works():
+    path = existing_csv(Path(__file__).parent / "fixtures" / "fake.csv")
+    assert path.name == "fake.csv"