add "help" command (#854)

fixes #787

src/popper/commands/cmd_help.py

@@ -0,0 +1,23 @@
+import click
+import os
+
+from popper.cli import pass_context, log, PopperCLI, cmd_folder
+
+popper_cli = PopperCLI()
+
+
+@click.command("help", short_help="shows help for a given command")
+@click.argument(
+    "subcommand",
+    type=click.Choice(popper_cli.list_commands(click.Context(popper_cli))),
+    required=False,
+)
+@pass_context
+def cli(ctx, subcommand):
+    """ Display help for a given command or popper default help
+    """
+    if subcommand:
+        target_command = popper_cli.get_command(ctx, subcommand)
+        log.info(target_command.get_help(click.Context(popper_cli)))
+    else:
+        log.info(popper_cli.get_help(click.Context(popper_cli)))

src/test/test_cmd_help.py

@@ -0,0 +1,30 @@
+from click.testing import CliRunner
+import click
+from click import Context
+
+from popper.commands import cmd_help
+from .test_common import PopperTest
+from popper.cli import PopperCLI, cmd_folder
+
+
+class TestCommandHelp(PopperTest):
+    def test_help(self):
+        popper_cli = PopperCLI()
+        subcommands = popper_cli.list_commands(click.Context(popper_cli))
+
+        help_texts = []
+        with self.assertLogs("popper") as test:
+            runner = CliRunner()
+            for subcommand in subcommands:
+                result = runner.invoke(cmd_help.cli, [subcommand])
+                self.assertEqual(result.exit_code, 0)
+                target_command = popper_cli.get_command(
+                    click.Context(popper_cli), subcommand
+                )
+                help_texts.append(target_command.get_help(click.Context(popper_cli)))
+
+        for command_output, test_output in zip(help_texts, test.output):
+            self.assertIn(
+                command_output.replace(" ", "").replace("\n", ""),
+                test_output.replace(" ", "").replace("\n", ""),
+            )