Print useful error message when filename is passed to stub/apply (#88)

Fixes #65.

monkeytype/cli.py

@@ -65,6 +65,11 @@ def complain_about_no_traces(args: argparse.Namespace, stderr: IO) -> None:
     module, qualname = args.module_path
     if qualname:
         print(f'No traces found for specifier {module}:{qualname}', file=stderr)
+    # When there is no trace and a top level module's filename is passed, print
+    # a useful error message.
+    elif os.path.exists(module):
+        print(f"No traces found for {module}; did you pass a filename instead of a module name? "
+              f"Maybe try just '{os.path.splitext(module)[0]}'.", file=stderr)
     else:
         print(f'No traces found for module {module}', file=stderr)
 

tests/test_cli.py

@@ -286,6 +286,24 @@ def test_pathlike_parameter(store_data, capsys):
         assert "test/foo.py does not look like a valid Python import path" in err
 
 
+def test_toplevel_filename_parameter(store_data, stdout, stderr):
+    filename = 'foo.py'
+    store, db_file = store_data
+    with mock.patch.dict(os.environ, {DefaultConfig.DB_PATH_VAR: db_file.name}):
+        orig_exists = os.path.exists
+
+        def side_effect(x):
+            return True if x == filename else orig_exists(x)
+        with mock.patch('os.path.exists', side_effect=side_effect) as mock_exists:
+            ret = cli.main(['stub', filename], stdout, stderr)
+            mock_exists.assert_called_with(filename)
+        err_msg = f"No traces found for {filename}; did you pass a filename instead of a module name? " \
+                  f"Maybe try just '{os.path.splitext(filename)[0]}'.\n"
+        assert stderr.getvalue() == err_msg
+        assert stdout.getvalue() == ''
+        assert ret == 0
+
+
 @pytest.mark.usefixtures("collector")
 def test_apply_stub_init(store_data, stdout, stderr, collector):
     """Regression test for applying stubs to testmodule/__init__.py style module layout"""