Click is designed to assist with the creation of complex and simple CLI tools alike. However, the power of its design is the ability to arbitrarily nest systems together. For instance, if you have ever used Django, you will have realized that it provides a command line utility, but so does Celery. When using Celery with Django, there are two tools that need to interact with each other and be cross-configured.
In a theoretical world of two separate Click command line utilities, they could solve this problem by nesting one inside the other. For instance, the web framework could also load the commands for the message queue framework.
To understand how this works, you need to understand two concepts: contexts and the calling convention.
Whenever a Click command is executed, a
Context object is created
which holds state for this particular invocation. It remembers parsed
parameters, what command created it, which resources need to be cleaned up
at the end of the function, and so forth. It can also optionally hold an
Context objects build a linked list until they hit the top one. Each context is linked to a parent context. This allows a command to work below another command and store its own information there without having to be afraid of altering up the state of the parent command.
Because the parent data is available, however, it is possible to navigate to it if needed.
Most of the time, you do not see the context object, but when writing more complex applications it comes in handy. This brings us to the next point.
When a Click command callback is executed, it’s passed all the non-hidden
parameters as keyword arguments. Notably absent is the context. However,
a callback can opt into being passed to the context object by marking itself
So how do you invoke a command callback if you don’t know if it should
receive the context or not? The answer is that the context itself
provides a helper function (
Context.invoke()) which can do this for
you. It accepts the callback as first argument and then invokes the
Building a Git Clone¶
In this example, we want to build a command line tool that resembles a version control system. Systems like Git usually provide one over-arching command that already accepts some parameters and configuration, and then have extra subcommands that do other things.
The Root Command¶
At the top level, we need a group that can hold all our commands. In this
case, we use the basic
click.group() which allows us to register
other Click commands below it.
For this command, we also want to accept some parameters that configure the state of our tool:
import os import click class Repo(object): def __init__(self, home=None, debug=False): self.home = os.path.abspath(home or '.') self.debug = debug @click.group() @click.option('--repo-home', envvar='REPO_HOME', default='.repo') @click.option('--debug/--no-debug', default=False, envvar='REPO_DEBUG') @click.pass_context def cli(ctx, repo_home, debug): ctx.obj = Repo(repo_home, debug)
Let’s understand what this does. We create a group command which can
have subcommands. When it is invoked, it will create an instance of a
Repo class. This holds the state for our command line tool. In this
case, it just remembers some parameters, but at this point it could also
start loading configuration files and so on.
This state object is then remembered by the context as
This is a special attribute where commands are supposed to remember what
they need to pass on to their children.
In order for this to work, we need to mark our function with
pass_context(), because otherwise, the context object would be
entirely hidden from us.
The First Child Command¶
Let’s add our first child command to it, the clone command:
@cli.command() @click.argument('src') @click.argument('dest', required=False) def clone(src, dest): pass
So now we have a clone command, but how do we get access to the repo? As
you can imagine, one way is to use the
pass_context() function which
again will make our callback also get the context passed on which we
memorized the repo. However, there is a second version of this decorator
pass_obj() which will just pass the stored object, (in our case
@cli.command() @click.argument('src') @click.argument('dest', required=False) @click.pass_obj def clone(repo, src, dest): pass
While not relevant for the particular program we want to build, there is
also quite good support for interleaving systems. Imagine for instance that
there was a super cool plugin for our version control system that needed a
lot of configuration and wanted to store its own configuration as
obj. If we would then attach another command below that,
we would all of a sudden get the plugin configuration instead of our repo
One obvious way to remedy this is to store a reference to the repo in the plugin, but then a command needs to be aware that it’s attached below such a plugin.
There is a much better system that can built by taking advantage of the linked nature of contexts. We know that the plugin context is linked to the context that created our repo. Because of that, we can start a search for the last level where the object stored by the context was a repo.
Built-in support for this is provided by the
factory, which will create decorators for us that find objects (it
internally calls into
Context.find_object()). In our case, we
know that we want to find the closest
Repo object, so let’s make a
decorator for this:
pass_repo = click.make_pass_decorator(Repo)
If we now use
pass_repo instead of
pass_obj, we will always get a
repo instead of something else:
@cli.command() @click.argument('src') @click.argument('dest', required=False) @pass_repo def clone(repo, src, dest): pass
Ensuring Object Creation¶
The above example only works if there was an outer command that created a
Repo object and stored it in the context. For some more advanced use
cases, this might become a problem. The default behavior of
make_pass_decorator() is to call
which will find the object. If it can’t find the object, it will raise an
error. The alternative behavior is to use
which will find the object, and if it cannot find it, will create one and
store it in the innermost context. This behavior can also be enabled for
make_pass_decorator() by passing
pass_repo = click.make_pass_decorator(Repo, ensure=True)
In this case, the innermost context gets an object created if it is missing. This might replace objects being placed there earlier. In this case, the command stays executable, even if the outer command does not run. For this to work, the object type needs to have a constructor that accepts no arguments.
As such it runs standalone:
@click.command() @pass_repo def cp(repo): click.echo(repo)
As you can see:
$ cp <Repo object at 0x7f6cafbb1810>