In addition to common functionality that is implemented in the library itself, there are countless patterns that can be implemented by extending Click. This page should give some insight into what can be accomplished.
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.
As explained in Custom Multi Commands, a multi command can provide
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 in order to avoid confusion.
This following example implements a subclass of
accepts a prefix for a command. If there were a command called
it would accept
pus as an alias (so long as it was 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) ctx.fail('Too many matches: %s' % ', '.join(sorted(matches)))
And it can then be used like this:
@click.command(cls=AliasedGroup) def cli(): pass @cli.command() def push(): pass @cli.command() def pop(): pass
Parameters (options and arguments) are forwarded to the command callbacks
as you have seen. One common way to prevent a parameter from being passed
to the callback is the expose_value argument to a parameter which hides
the parameter entirely. The way this works is that the
object has a
params attribute which is a dictionary of
all parameters. Whatever is in that dictionary is being passed to the
This can be used to make up addition parameters. Generally this pattern is not recommended but in some cases it can be useful. At the very least it’s good to know that the system works this way.
import urllib def open_url(ctx, param, value): if value is not None: ctx.params['fp'] = urllib.urlopen(value) return value @click.command() @click.option('--url', callback=open_url) def cli(url, fp=None): if fp is not None: click.echo('%s: %s' % (url, fp.code))
In this case the callback returns the URL unchanged but also passes a
fp value to the callback. What’s more recommended is to pass
the information in a wrapper however:
import urllib class URL(object): def __init__(self, url, fp): self.url = url self.fp = fp def open_url(ctx, param, value): if value is not None: return URL(value, urllib.urlopen(value)) @click.command() @click.option('--url', callback=open_url) def cli(url): if url is not None: click.echo('%s: %s' % (url.url, url.fp.code))
New in version 2.0.
Starting with Click 2.0, it’s possible to provide a function that is used for normalizing tokens. Tokens are option names, choice values, or command values. This can be used to implement case insensitive options, for instance.
In order to use this feature, the context needs to be passed a function that performs the normalization of the token. For instance, you could have a function that converts the token to lowercase:
CONTEXT_SETTINGS = dict(token_normalize_func=lambda x: x.lower()) @click.command(context_settings=CONTEXT_SETTINGS) @click.option('--name', default='Pete') def cli(name): click.echo('Name: %s' % name)
And how it works on the command line:
$ cli --NAME=Pete Name: Pete
Invoking Other Commands¶
Sometimes, it might be interesting to invoke one command from another
command. This is a pattern that is generally discouraged with Click, but
possible nonetheless. For this, you can use the
They work similarly, but the difference is that
invokes another command with the arguments you provide as a caller,
Context.forward() fills in the arguments from the current
command. Both accept the command as the first argument and everything else
is passed onwards as you would expect.
cli = click.Group() @cli.command() @click.option('--count', default=1) def test(count): click.echo('Count: %d' % count) @cli.command() @click.option('--count', default=1) @click.pass_context def dist(ctx, count): ctx.forward(test) ctx.invoke(test, count=42)
And what it looks like:
$ cli dist Count: 1 Count: 42
Callback Evaluation Order¶
Click works a bit differently 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 porting complex patterns to Click from optparse or other systems. 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; if there is an option called
and an option called
--bar and the user calls it as
--foo, then the callback for
bar will fire before the one for
There are three 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
--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 is repeated – for instance,
--exclude foo --include baz --exclude bar– the callback will fire based on the position of the first option. In this case, the callback will fire for
excludeand it will be passed both options (
bar), then the callback for
includewill fire with
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 last. 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 do not need to be concerned about any of this, but it is important to know how it works for some advanced cases.
Forwarding Unknown Options¶
In some situations it is interesting to be able to accept all unknown
options for further manual processing. Click can generally do that as of
Click 4.0, but it has some limitations that lie in the nature of the
problem. The support for this is provided through a parser flag called
ignore_unknown_options which will instruct the parser to collect all
unknown options and to put them to the leftover argument instead of
triggering a parsing error.
This can generally be activated in two different ways:
- It can be enabled on custom
Commandsubclasses by changing the
- It can be enabled by changing the attribute of the same name on the
context class (
Context.ignore_unknown_options). This is best changed through the
context_settingsdictionary on the command.
For most situations the easiest solution is the second. Once the behavior is changed something needs to pick up those leftover options (which at this point are considered arguments). For this again you have two options:
- You can use
pass_context()to get the context passed. This will only work if in addition to
ignore_unknown_optionsyou also set
allow_extra_argsas otherwise the command will abort with an error that there are leftover arguments. If you go with this solution, the extra arguments will be collected in
- You can attach a
nargsset to -1 which will eat up all leftover arguments. In this case it’s recommeded to set the type to
UNPROCESSEDto avoid any string processing on those arguments as otherwise they are forced into unicode strings automatically which is often not what you want.
In the end you end up with something like this:
import sys from subprocess import call @click.command(context_settings=dict( ignore_unknown_options=True, )) @click.option('-v', '--verbose', is_flag=True, help='Enables verbose mode') @click.argument('timeit_args', nargs=-1, type=click.UNPROCESSED) def cli(verbose, timeit_args): """A wrapper around Python's timeit.""" cmdline = ['python', '-mtimeit'] + list(timeit_args) if verbose: click.echo('Invoking: %s' % ' '.join(cmdline)) call(cmdline)
And what it looks like:
$ cli --help Usage: cli [OPTIONS] [TIMEIT_ARGS]... A wrapper around Python's timeit. Options: -v, --verbose Enables verbose mode --help Show this message and exit. $ cli -n 100 "a = 1; b = 2; a * b" 100 loops, best of 3: 0.0286 usec per loop $ cli -v "a = 1; b = 2; a * b" Invoking: python -mtimeit a = 1; b = 2; a * b 10000000 loops, best of 3: 0.0285 usec per loop
As you can see the verbosity flag is handled by Click, everything else ends up in the timeit_args variable for further processing which then for instance, allows invoking a subprocess. There are a few things that are important to know about how this ignoring of unhandled flag happens:
- Unknown long options are generally ignored and not processed at all.
So for instance if
--foo barare passed they generally end up like that. Note that because the parser cannot know if an option will accept an argument or not, the
barpart might be handled as an argument.
- Unknown short options might be partially handled and reassmebled if
necessary. For instance in the above example there is an option
-vwhich enables verbose mode. If the command would be ignored with
-vpart would be handled by Click (as it is known) and
-awould end up in the leftover parameters for further processing.
- Depending on what you plan on doing you might have some success by
disabling interspersed arguments
allow_interspersed_args) which instructs the parser to not allow arguments and options to be mixed. Depending on your situation this might improve your results.
Generally though the combinated handling of options and arguments from your own commands and commands from another application are discouraged and if you can avoid it, you should. It’s a much better idea to have everything below a subcommand be forwarded to another application than to handle some arguments yourself.
Global Context Access¶
New in version 5.0.
Starting with Click 5.0 it is possible to access the current context from
anywhere within the same through through the use of the
get_current_context() function which returns it. This is primarily
useful for accessing the context bound object as well as some flags that
are stored on it to customize the runtime behavior. For instance the
echo() function does this to infer the default value of the color
def get_current_command_name(): return click.get_current_context().info_name
It should be noted that this only works within the current thread. If you spawn additional threads then those threads will not have the ability to refer to the current context. If you want to give another thread the ability to refer to this context you need to use the context within the thread as a context manager:
def spawn_thread(ctx, func): def wrapper(): with ctx: func() t = threading.Thread(target=wrapper) t.start() return t
Now the thread function can access the context like the main thread would do. However if you do use this for threading you need to be very careful as the vast majority of the context is not thread safe! You are only allowed to read from the context, but not to perform any modifications on it.