Better usage of `make_pass_decorator` in Python Click

Question

I am looking for some advice to avoid having to instantiate a class twice; this is more of a design pattern question. I am creating an application using the Python Click library.

I have a Settings class that first loads all initial default settings into a dictionary (hard-coded into the application), then loads all settings overrides (if specified) from a TOML file on the user's computer into a dictionary, and then finally merges the two and makes them available as attributes of the class instance (settings.<something>).

For most of these settings, I also want to be able to specify a command-line flag. The priority then becomes:

1. Command-line flag. If not specified, then fallback to...
2. User setting in TOML file. If not specified, then finally fallback to...
3. Application default

In order to achieve this result, I am finding that, when using Click's decorators, I have to do something like this:

import click
from myapp import Settings

settings = Settings()
pass_settings = click.make_pass_decorator(Settings, ensure=True)

@click.command()
@click.help_option('-h', '--help')
@click.option(
    '-s', '--disk-size',
    default=settings.instance_disk_size,
    help="Disk size",
    show_default=True,
    type=int
)
@click.option(
    '-t', '--disk-type',
    default=settings.instance_disk_type,
    help="Disk type",
    show_default=True,
    type=click.Choice(['pd-standard', 'pd-ssd'])
)
@pass_settings
def create(settings, disk_size, disk_type):
    print(disk_size)
    print(disk_type)

Why twice?

• The settings = Settings() line is needed to provide the @click.option decorators with the default value. The default value could either come from the user override TOML file (if present), or from the application default.
• The click.make_pass_decorator seems to be the recommended way for interleaved commands; it's even mentioned in their documentation. Inside of the function, in addition to the CLI parameters passed, I also sometimes needs to reference other attributes in the Settings class.

My question is, which is better? Is there a way to use the pass_settings decorator in the other click.option decorators? Or should I ditch using click.make_pass_decorator entirely?

Answer 1

One way to approach the problem of not wanting to instantiate Settings twice, is to inherit from click.Option, and insert the settings instance into the context directly like:

Custom Class:

def build_settings_option_class(settings_instance):

    def set_default(default_name):

        class Cls(click.Option):
            def __init__(self, *args, **kwargs):
                kwargs['default'] = getattr(settings_instance, default_name)
                super(Cls, self).__init__(*args, **kwargs)

            def handle_parse_result(self, ctx, opts, args):
                obj = ctx.find_object(type(settings_instance))
                if obj is None:
                    ctx.obj = settings_instance

                return super(Cls, self).handle_parse_result(ctx, opts, args)

        return Cls

    return set_default
    

Using Custom Class:

To use the custom class, pass the cls parameter to @click.option() decorator like:

# instantiate settings
settings = Settings()

# get the setting option builder
settings_option_cls = build_settings_option_class(settings)

# decorate with an option with an appropraie option name
@click.option("--an_option", cls=settings_option_cls('default_setting_name'))

How does this work?

This works because click is a well designed OO framework. The @click.option() decorator usually instantiates a click.Option object but allows this behavior to be over ridden with the cls parameter. So it is a relatively easy matter to inherit from click.Option in our own class and over ride the desired methods.

In this case we use a couple of closures to capture the Settings instance and parameter name. In the returned class we over ride click.Option.handle_parse_result() to allow us to insert the setting object into the context. This allows the pass_settings decorator to find the settings in the context, and thus it will not need to create a new instance.

Test Code:

import click

class Settings(object):

    def __init__(self):
        self.instance_disk_size = 100
        self.instance_disk_type = 'pd-ssd'


settings = Settings()
settings_option_cls = build_settings_option_class(settings)
pass_settings = click.make_pass_decorator(Settings)


@click.command()
@click.help_option('-h', '--help')
@click.option(
    '-s', '--disk-size',
    cls=settings_option_cls('instance_disk_size'),
    help="Disk size",
    show_default=True,
    type=int
)
@click.option(
    '-t', '--disk-type',
    cls=settings_option_cls('instance_disk_type'),
    help="Disk type",
    show_default=True,
    type=click.Choice(['pd-standard', 'pd-ssd'])
)
@pass_settings
def create(settings, disk_size, disk_type):
    print(disk_size)
    print(disk_type)


if __name__ == "__main__":
    commands = (
        '-t pd-standard -s 200',
        '-t pd-standard',
        '-s 200',
        '',
        '--help',
    )

    import sys, time

    time.sleep(1)
    print('Click Version: {}'.format(click.__version__))
    print('Python Version: {}'.format(sys.version))
    for cmd in commands:
        try:
            time.sleep(0.1)
            print('-----------')
            print('> ' + cmd)
            time.sleep(0.1)
            create(cmd.split())

        except BaseException as exc:
            if str(exc) != '0' and \
                    not isinstance(exc, (click.ClickException, SystemExit)):
                raise
                

Test Results:

Click Version: 6.7
Python Version: 3.6.2 (default, Jul 17 2017, 23:14:31) 
[GCC 5.4.0 20160609]
-----------
> -t pd-standard -s 200
200
pd-standard
-----------
> -t pd-standard
100
pd-standard
-----------
> -s 200
200
pd-ssd
-----------
> 
100
pd-ssd
-----------
> --help
Usage: test.py [OPTIONS]

Options:
  -h, --help                      Show this message and exit.
  -s, --disk-size INTEGER         Disk size  [default: 100]
  -t, --disk-type [pd-standard|pd-ssd]
                                  Disk type  [default: pd-ssd]