fix help message display for dags test subcommand

[houqp]

Without this fix, airflow dags test crashes with the following error:

    item_help = join([func(*args) for func, args in self.items])
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py", line 531, in _format_action
    help_text = self._expand_help(action)
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py", line 620, in _expand_help
    return self._get_help_string(action) % params
TypeError: unsupported operand type(s) for %: 'tuple' and 'dict'

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[mik-laj]

Can you add test to avoid regression? I think something similar to the one below is enough.

tests/cli/test_cli_parser.py

    def test_should_display_helps(self):
        parser = cli_parser.get_parser()

        all_command_as_args = sum([
            [[top_commaand.name]]
            if isinstance(top_commaand, cli_parser.ActionCommand)
            else [
                [top_commaand.name, nested_command.name] for nested_command in top_commaand.subcommands
            ]

            for top_commaand in cli_parser.airflow_commands
        ], [])
        for cmd_args in all_command_as_args:
            with self.assertRaises(SystemExit):
                parser.parse_args([*cmd_args, '--help'])

[zhongjiajie]

Thanks, LGTM, but still need add test as Kamil said

[zhongjiajie]

BTW, Kamil test is awesome.

[turbaszek]

FYI flat map suggest by Kamil isn't best in terms of performance but it doesn't matter here probably.
https://dev.to/turbaszek/flat-map-in-python-3g98

[kaxil]

FYI flat map suggest by Kamil isn't best in terms of performance but it doesn't matter here probably.
https://dev.to/turbaszek/flat-map-in-python-3g98

Nice blog post @turbaszek

[potiuk]

I merged it too quick - did not notice the last comment but indeed @hoqp -> tests would be awesome :)

[houqp]

@mik-laj @zhongjiajie @potiuk please see PR #8561 for unit test.

Also thanks @turbaszek for the optimization tip. Like you said not going to matter in this case, but it's good to know. I changed mik-laj's implementation to use nested list comprehension based on the blog post you shared.