Python Click: custom error message

Question

I use the excellent Python Click library for handling command line options in my tool. Here\'s a simplified version of my code (full script here):

@click.com         


        

Answer 1

Message construction for most errors in python-click is handled by the show method of the UsageError class: click.exceptions.UsageError.show.

So, if you redefine this method, you will be able to create your own customized error message. Below is an example of a customization which appends the help menu to any error message which answers this SO question:

def modify_usage_error(main_command):
    '''
        a method to append the help menu to an usage error

    :param main_command: top-level group or command object constructed by click wrapper 
    :return: None
    '''

    from click._compat import get_text_stderr
    from click.utils import echo
    def show(self, file=None):
        import sys
        if file is None:
            file = get_text_stderr()
        color = None
        if self.ctx is not None:
            color = self.ctx.color
            echo(self.ctx.get_usage() + '\n', file=file, color=color)
        echo('Error: %s\n' % self.format_message(), file=file, color=color)
        sys.argv = [sys.argv[0]]
        main_command()

    click.exceptions.UsageError.show = show

Once you define your main command, you can then run the modifier script:

import click
@click.group()
def cli():
    pass

modify_usage_error(cli)

I have not explored whether there are runtime invocations of ClickException other than usage errors. If there are, then you might need to modify your custom error handler to first check that ctx is an attribute before you add the line click.exceptions.ClickException.show = show since it does not appear that ClickException is fed ctx at initialization.