testing: add CapturingTestCase to test stdout/stderr/click CLI

tests/base.py

@@ -20,6 +20,30 @@ def setUpClass(cls):
     def tearDown(self):
         initLogging()
 
+class CapturingTestCase(TestCase):
+    """
+    A TestCase that needs to capture stderr/stdout and invoke click CLI.
+    """
+
+    @pytest.fixture(autouse=True)
+    def _setup_pytest_capfd(self, capfd):
+        self.capfd = capfd
+
+    def invoke_cli(self, cli, args):
+        """
+        Substitution for click.CliRunner.invooke that works together nicely
+        with unittests/pytest capturing stdout/stderr.
+        """
+        self.capture_out_err()  # XXX snapshot just before executing the CLI
+        try:
+            cli.main(args=args)
+        except SystemExit as e:
+            out, err = self.capture_out_err()
+            return e.code, out, err
+
+    def capture_out_err(self):
+        return self.capfd.readouterr()
+
 #  import traceback
 #  import warnings
 #  def warn_with_traceback(message, category, filename, lineno, file=None, line=None):