Where are Errors Handled?¶

Click’s main error handling is happening in Command.main(). In there it handles all subclasses of ClickException as well as the standard EOFError and KeyboardInterrupt exceptions. The latter are internally translated into an Abort.

The logic applied is the following:

1. If an EOFError or KeyboardInterrupt happens, reraise it as Abort.

2. If a ClickException is raised, invoke the ClickException.show() method on it to display it and then exit the program with ClickException.exit_code.

3. If an Abort exception is raised print the string Aborted! to standard error and exit the program with exit code 1.

4. If it goes through well, exit the program with exit code 0.

What if I Don’t Want That?¶

Generally you always have the option to invoke the Command.invoke() method yourself. For instance if you have a Command you can invoke it manually like this:

ctx = command.make_context("command-name", ["args", "go", "here"])
with ctx:
    result = command.invoke(ctx)

In this case exceptions will not be handled at all and bubbled up as you would expect.

Starting with Click 3.0 you can also use the Command.main() method but disable the standalone mode which will do two things: disable exception handling and disable the implicit sys.exit() at the end.

So you can do something like this:

command.main(
    ["command-name", "args", "go", "here"],
    standalone_mode=False,
)

Which Exceptions Exist?¶

Click has two exception bases: ClickException which is raised for all exceptions that Click wants to signal to the user and Abort which is used to instruct Click to abort the execution.

A ClickException has a ClickException.show() method which can render an error message to stderr or the given file object. If you want to use the exception yourself for doing something check the API docs about what else they provide.

The following common subclasses exist:

• UsageError to inform the user that something went wrong.

• BadParameter to inform the user that something went wrong with a specific parameter. These are often handled internally in Click and augmented with extra information if possible. For instance if those are raised from a callback Click will automatically augment it with the parameter name if possible.

• FileError this is an error that is raised by the FileType if Click encounters issues opening the file.

Help Pages and Exit Codes¶

Triggering the a help page intentionally (by passing in --help) returns exit code 0. If a help page is displayed due to incorrect user input, the program returns exit code 2. See Exit Codes for more general information.

For clarity, here is an example.

@click.group('printer_group')
def printer_group():
    pass

@printer_group.command('printer')
@click.option('--this')
def printer(this):
    if this:
        click.echo(this)

$ printer-group --help
Usage: printer-group [OPTIONS] COMMAND [ARGS]...

Options:
  --help  Show this message and exit.

Commands:
  printer

The above invocation returns exit code 0.

$ printer-group
Usage: printer-group [OPTIONS] COMMAND [ARGS]...

Options:
  --help  Show this message and exit.

Commands:
  printer

The above invocation returns exit code 2 since the user invoked the command incorrectly. However, since this is such a common error when first using a command, Click invokes the help page for the user. To see that printer-group is an invalid invocation, turn no_args_is_help off.

@click.group('printer_group', no_args_is_help=False)
def printer_group():
    pass

@printer_group.command('printer')
@click.option('--this')
def printer(this):
    if this:
        click.echo(this)

$ printer-group
Usage: printer-group [OPTIONS] COMMAND [ARGS]...
Try 'printer-group --help' for help.

Error: Missing command.