Require an option only if a choice is made when using click

Question

When using click I know how to define a multiple choice option. I also know how to set an option as a required one. But, how can I indicate that an option B is required only if the value of option A is foo?

Here's an example:

import click
@click.command()
@click.option('--output',
 type=click.Choice(['stdout', 'file']), default='stdout')
@click.option('--filename', type=click.STRING)
def main(output, filename):
 print("output: " + output)
 if output == 'file':
 if filename is None:
 print("filename must be provided!")
 else:
 print("filename: " + str(filename))
if __name__ == "__main__":
 main()

If the output option is stdout, then filename is not needed. However, if the user chooses output to be file, then the other option filename must be provided. Is this pattern supported by click?

At the beginning of the function I can add something like:

if output == 'file' and filename is None:
 raise ValueError('When output is "file", a filename must be provided')

But I am interested whether there's a nicer/cleaner solution.

Answer 1

In the particular case of this example, I think an easier method would be to get rid of --output, and simply assume stdout if --filename is not specified and if --filename is specified, then use it instead of stdout.

But assuming this is a contrived example, you can inherit from click.Option to allow hooking into the click processing:

Custom Class:

class OptionRequiredIf(click.Option):
 def full_process_value(self, ctx, value):
 value = super(OptionRequiredIf, self).full_process_value(ctx, value)
 if value is None and ctx.params['output'] == 'file':
 msg = 'Required if --output=file'
 raise click.MissingParameter(ctx=ctx, param=self, message=msg)
 return value

Using Custom Class:

To use the custom class, pass it as the cls argument to the option decorator like:

@click.option('--filename', type=click.STRING, cls=OptionRequiredIf)

Test Code:

import click
@click.command()
@click.option('--output',
 type=click.Choice(['stdout', 'file']), default='stdout')
@click.option('--filename', type=click.STRING, cls=OptionRequiredIf)
def main(output, filename):
 print("output: " + output)
 if output == 'file':
 if filename is None:
 print("filename must be provided!")
 else:
 print("filename: " + str(filename))
main('--output=file'.split())

Results:

Usage: test.py [OPTIONS]
Error: Missing option "--filename". Required if --output=file

• Dror: This seems to be very elegant solution. I tried to modify it a little and provide two more parameters to the class OptionRequiredIf. One would be depends_on_opt which in this example would be output and the other depends_on_val which would hold file. I failed...
• Dror: I'm afraid this solution doesn't work... When providing the filename it is not populating the corresponding variable. I posted a complete example on gist
• Stephen Rauch: @dror, sorry I missed the return value, I was getting late for work, so I didn't test as well as I should have. Glad it worked out and thanks for the edit...

Answer 2

You can do the same thing with a custom validation callback:

import click
def required_with_output(ctx, param, value):
 if ctx.params.get("output") != "stdout" and value is None:
 raise click.BadParameter("--output requires --filename")
 return value
@click.command()
@click.option(
 "--output",
 type=click.Choice(["stdout", "file"]),
 default="stdout",
)
@click.option("--filename", callback=required_with_output)
def main(output, filename):
 print("output: " + output)
 if output == "file":
 if filename is None:
 print("filename must be provided!")
 else:
 print("filename: " + str(filename))
if __name__ == "__main__":
 main()

I think this is a little simpler.

Answer 3

I extended the answer by Stephen, and made it more generic:

class OptionRequiredIf(click.Option):
 """
 Option is required if the context has `option` set to `value`
 """
 def __init__(self, *a, **k):
 try:
 option = k.pop('option')
 value = k.pop('value')
 except KeyError:
 raise(KeyError("OptionRequiredIf needs the option and value "
 "keywords arguments"))
 click.Option.__init__(self, *a, **k)
 self._option = option
 self._value = value
 def process_value(self, ctx, value):
 value = super(OptionRequiredIf, self).process_value(ctx, value)
 if value is None and ctx.params[self._option] == self._value:
 msg = 'Required if --{}={}'.format(self._option, self._value)
 raise click.MissingParameter(ctx=ctx, param=self, message=msg)
 return value

Usage example:

@click.option('--email', type=click.STRING,
 help='Settings for sending emails.',
 option='output', value='email', cls=OptionRequiredIf)

I was inspired by this answer