Advanced Patterns

In addition to common functionality that is implemented in the library itself, there are countless of patterns that can be implemented by extending click. This page should give some inspiration of what can be accomplished.

Command Aliases

Many tools support aliases for commands. For instance you can configure git to accept git ci as alias for git commit. Other tools also support auto discovery for aliases by automatically shortening them.

Click does not support this out of the box but it’s very easy to customize the Group or any other MultiCommand to provide this functionality.

As explained in Custom Multi Commands a multi command can provide two methods: list_commands() and get_command(). In this particular case you only need to override the latter as you generally don’t want to enumerate the aliases on the help page to avoid confusion.

This following example implements a subclass of Group that accepts a prefix for a command. So if there is a command called push it would accept pus as alias if it’s unique:

class AliasedGroup(click.Group):

    def get_command(self, ctx, cmd_name):
        rv = click.Group.get_command(self, ctx, cmd_name)
        if rv is not None:
            return rv
        matches = [x for x in self.list_commands(ctx)
                   if x.startswith(cmd_name)]
        if not matches:
            return None
        elif len(matches) == 1:
            return click.Group.get_command(self, ctx, matches[0])'Too many matches: %s' % ', '.join(sorted(matches)))

And it can be used like this then:

def cli():

def push():

def pop():

Invoking Other Commands

Sometimes it might be interesting to invoke one command from another command. This is generally a pattern that is discouraged with click but possible nonetheless. For this you can use the Context.invoke() or Context.forward() methods.

They work similar but the difference is that Context.invoke() merely invokes another command with the arguments you provide as a caller, whereas Context.forward() fills in the arguments from the current command. Both accept the command as first argument and everything else is passed onwards as you would expect.


cli = click.Group()

@click.option('--count', default=1)
def test(count):
    click.echo('Count: %d' % count)

@click.option('--count', default=1)
def dist(ctx, count):
    ctx.invoke(test, count=42)

And what it looks like:

$ cli dist
Count: 1
Count: 42

Showing Long Texts

In some situations you might want to show long texts on the terminal and let a user scroll through it. This can be achieved by using the echo_via_pager() function which works similar to the echo() function but always writes to stdout and, if possible, through a pager.


def less():
    click.echo_via_pager('\n'.join('Line %d' % idx
                                   for idx in range(200)))

Callback Evaluation Order

Click works a bit different than some other command line parsers in that it attempts to reconcile the order of arguments as defined by the programmer with the order of arguments as defined by the user before invoking any callbacks.

This is an important concept to understand when implementing complex patterns ported from optparse or other systems to click. A parameter callback invocation in optparse happens as part of the parsing step whereas a callback invocation in click happens after the parsing.

The main difference is that in optparse callbacks are invoked with the raw value as it happens, whereas a callback in click is invoked after the value has been fully converted.

Generally the order of invocation is driven by the order in which the user provides the arguments to the script. So if there is an option called --foo and an option called --bar and the user calls it as --bar --foo then the callback for bar will fire before the one for foo.

There are two exceptions to this rule which are important to know:


An option can be set to be “eager”. All eager parameters are evaluated before all non-eager parameters, but again in the order as they were provided on the command line by the user.

This is important for parameters that execute and exit like --help and --version. Both are eager parameters but whatever parameter comes first on the command line will win and exit the program.

Repeated parameters:

If an option or argument is split up on the command line into multiple places because it’s repeated (for instance --exclude foo --include baz --exclude bar) the callback will fire based on the position of the first option. So in this case the callback will fire for exclude and it will be passed both options (foo and bar), then the callback for include will fire with baz only.

Note that even if a parameter does not allow multiple versions, click will still accept the position of the first, but it will ignore every value except the first. The reason for this is to allow composability through shell aliases that set defaults.

Missing parameters:

If a parameter is not defined on the command line the callback will still fire. This is different from how it works in optparse where undefined values do not fire the callback. Missing parameters fire their callbacks at the very end which makes it possible for them to default to values from a parameter that came before.

Most of the time you don’t need to be concerned about any of this stuff, but it’s important to know how it works for some advanced cases.