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https://github.com/jbranchaud/til
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jbranchaud
54 lines
2.3 KiB
Markdown
54 lines
2.3 KiB
Markdown
# Check Precondition Before Click Arg Parsing
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When setting up various [Click](https://click.palletsprojects.com/en/stable/)
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subcommands with options, I ran into an issue with the order of some validation
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checks. I was putting the same precondition validation logic at the beginning of
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several subcommands. I was also putting callback validations on specific options
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to those subcommands. Ideally the option validations could rely on those
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precondition validations. However, the option callbacks run before anything in
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the body of the subcommands.
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The solution was to move those preconditions out of the subcommand body
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(simplifying the subcommand) and into a `click.Command` subclass.
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To demonstrate that, I'll first show the `click.Command` subclass:
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```python
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class RequireActiveSessionCommand(click.Command):
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def parse_args(self, ctx, args):
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if ctx.obj.active_session is None:
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msg = "No active session being tracked. Start a session first."
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raise click.UsageError(msg)
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return super().parse_args(ctx, args)
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```
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The only thing this subclass overrides is `parse_args` where it gets ahead of
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the standard arg parsing logic to first check the precondition. In this case, I
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check that there is an active session. If there isn't, then I can raise a
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`click.UsageError`. Otherwise, it delegates back to the super-class
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implementation of `parse_args`.
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This subclass then gets used for the commands that need to enforce this
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precondition. Two prime examples of that are the `stop` and `cancel` subcommands.
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```python
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@cli.command(cls=RequireActiveSessionCommand)
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@click.option("--at", help='Hours previous to end the timer, e.g. "2 hours ago"', callback=validate_stop_at)
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@pass_cli
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def stop(cli_ctx: CliContext, at: datetime) -> None:
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# ... implementation omitted
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@cli.command(cls=RequireActiveSessionCommand)
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@pass_cli
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def cancel(cli_ctx: CliContext):
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# ... implementation omitted
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```
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Other subcommands, like `start` and `status` that don't need to enforce this
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precondition use the `@cli.command()` decorator without passing in a custom
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subclass.
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This example is pulled directly from [this commit](https://github.com/jbranchaud/py-vmt/commit/505109b7a4013e05f085cded666c6b1ac7c3c250)
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of my [`py-vmt` time tracker tool](https://github.com/jbranchaud/py-vmt).
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