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jbranchaud
65 lines
2.2 KiB
Markdown
65 lines
2.2 KiB
Markdown
# Validate Click Option With Callback
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I have a [click](https://click.palletsprojects.com/en/stable/) subcommand in my
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[`py-vmt` project](https://github.com/jbranchaud/py-vmt) that includes an
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`option` specified with the `--at` flag. This is what it originally looked like:
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```python
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# define `start` subcommand
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@cli.command()
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@click.argument("project-name")
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@click.option("--at", help='Relative time in past to start the time, e.g. "2 hours ago", "33 minutes ago"')
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@pass_cli
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def start(cli_ctx: CliContext, project_name: str, at: str | None) -> None:
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# ...
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```
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The value of `at` needs to be in the past. I need a way validate that it is or
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otherwise bail early with a useful error message. The optional
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[`callback`](https://click.palletsprojects.com/en/stable/advanced/#callbacks-for-validation)
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to `@click.option` plus `click.BadParameter` are a good way to handle that.
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First, I define a callback handler that does the validation. I even take it a
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step further and have it return the transformed value (`datetime`) that the
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subcommand logic will need.
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```python
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def validate_past_time(_ctx, _param, value: str | None) -> datetime:
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now = datetime.now(timezone.utc)
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if value == None:
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return now
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start_at = time_helpers.parse_to_datetime(value)
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if start_time == None or start_at > now:
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raise click.BadParameter("must be a relative time in the past")
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return start_at
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```
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I ignore the first two arguments because I only need to work with `value`. Value
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might be something like `"33 minutes ago"` and I attempt to transform that with
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`dateparser` into a `datetime` instance. If it can't be parsed or it isn't in
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the past, then I raise `click.BadParameter` which presents the user with useful
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usage details.
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This callback can then be incorporated into the subcommand like so:
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```python
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# define `start` subcommand
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@cli.command()
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@click.argument("project-name")
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@click.option(
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"--at",
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help='Relative time in past to start the time, e.g. "2 hours ago", "33 minutes ago"',
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callback=validate_past_time
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)
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@pass_cli
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def start(cli_ctx: CliContext, project_name: str, at: datetime) -> None:
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# ...
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```
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Now I can expect the incoming `at` option to be a `datetime` which helps
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simplify several lines of logic in the `start` implementation.
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