Quickstart
==========

.. currentmodule:: click

Install
----------------------
Install from PyPI::

    pip install click

Installing into a virtual environment is highly recommended. We suggest :ref:`virtualenv-heading`.

Examples
-----------------------

Some standalone examples of Click applications are packaged with Click. They are available in the `examples folder <https://github.com/pallets/click/tree/main/examples>`_ of the repo.

*   `inout <https://github.com/pallets/click/tree/main/examples/inout>`_ : A very simple example of an application that can read from files and write to files and also accept input from stdin or write to stdout.
*   `validation <https://github.com/pallets/click/tree/main/examples/validation>`_ : A simple example of an application that performs custom validation of parameters in different ways.
*   `naval <https://github.com/pallets/click/tree/main/examples/naval>`_ : Port of the `docopt <http://docopt.org/>`_ naval example.
*   `colors <https://github.com/pallets/click/tree/main/examples/colors>`_ : A simple example that colorizes text. Uses colorama on Windows.
*   `aliases <https://github.com/pallets/click/tree/main/examples/aliases>`_ : An advanced example that implements :ref:`aliases`.
*   `imagepipe <https://github.com/pallets/click/tree/main/examples/imagepipe>`_ : A complex example that implements some :ref:`multi-command-chaining` . It chains together image processing instructions. Requires pillow.
*   `repo <https://github.com/pallets/click/tree/main/examples/repo>`_ : An advanced example that implements a Git-/Mercurial-like command line interface.
*   `complex <https://github.com/pallets/click/tree/main/examples/complex>`_ : A very advanced example that implements loading subcommands dynamically from a plugin folder.
*   `termui <https://github.com/pallets/click/tree/main/examples/termui>`_ : A simple example that showcases terminal UI helpers provided by click.

Basic Concepts - Creating a Command
-----------------------------------

Click is based on declaring commands through decorators.  Internally, there
is a non-decorator interface for advanced use cases, but it's discouraged
for high-level usage.

A function becomes a Click command line tool by decorating it through
:func:`click.command`.  At its simplest, just decorating a function
with this decorator will make it into a callable script:

.. click:example::

    import click

    @click.command()
    def hello():
        click.echo('Hello World!')

What's happening is that the decorator converts the function into a
:class:`Command` which then can be invoked::

    if __name__ == '__main__':
        hello()

And what it looks like:

.. click:run::

    invoke(hello, args=[], prog_name='python hello.py')

And the corresponding help page:

.. click:run::

    invoke(hello, args=['--help'], prog_name='python hello.py')

Echoing
-------

Why does this example use :func:`echo` instead of the regular
:func:`print` function?  The answer to this question is that Click
attempts to support different environments consistently and to be very
robust even when the environment is misconfigured.  Click wants to be
functional at least on a basic level even if everything is completely
broken.

What this means is that the :func:`echo` function applies some error
correction in case the terminal is misconfigured instead of dying with a
:exc:`UnicodeError`.

The echo function also supports color and other styles in output. It
will automatically remove styles if the output stream is a file. On
Windows, colorama is automatically installed and used. See
:ref:`ansi-colors`.

If you don't need this, you can also use the `print()` construct /
function.

Nesting Commands
----------------

Commands can be attached to other commands of type :class:`Group`.  This
allows arbitrary nesting of scripts.  As an example here is a script that
implements two commands for managing databases:

.. click:example::

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

    @click.command()
    def initdb():
        click.echo('Initialized the database')

    @click.command()
    def dropdb():
        click.echo('Dropped the database')

    cli.add_command(initdb)
    cli.add_command(dropdb)

As you can see, the :func:`group` decorator works like the :func:`command`
decorator, but creates a :class:`Group` object instead which can be given
multiple subcommands that can be attached with :meth:`Group.add_command`.

For simple scripts, it's also possible to automatically attach and create a
command by using the :meth:`Group.command` decorator instead.  The above
script can instead be written like this:

.. click:example::

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

    @cli.command()
    def initdb():
        click.echo('Initialized the database')

    @cli.command()
    def dropdb():
        click.echo('Dropped the database')

You would then invoke the :class:`Group` in your setuptools entry points or
other invocations::

    if __name__ == '__main__':
        cli()


Registering Commands Later
--------------------------

Instead of using the ``@group.command()`` decorator, commands can be
decorated with the plain ``@click.command()`` decorator and registered
with a group later with ``group.add_command()``. This could be used to
split commands into multiple Python modules.

.. code-block:: python

    @click.command()
    def greet():
        click.echo("Hello, World!")

.. code-block:: python

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

    group.add_command(greet)


Adding Parameters
-----------------

To add parameters, use the :func:`option` and :func:`argument` decorators:

.. click:example::

    @click.command()
    @click.option('--count', default=1, help='number of greetings')
    @click.argument('name')
    def hello(count, name):
        for x in range(count):
            click.echo(f"Hello {name}!")

What it looks like:

.. click:run::

    invoke(hello, args=['--help'], prog_name='python hello.py')

.. _switching-to-setuptools:

Switching to Setuptools
-----------------------

In the code you wrote so far there is a block at the end of the file which
looks like this: ``if __name__ == '__main__':``.  This is traditionally
how a standalone Python file looks like.  With Click you can continue
doing that, but there are better ways through setuptools.

There are two main (and many more) reasons for this:

The first one is that setuptools automatically generates executable
wrappers for Windows so your command line utilities work on Windows too.

The second reason is that setuptools scripts work with virtualenv on Unix
without the virtualenv having to be activated.  This is a very useful
concept which allows you to bundle your scripts with all requirements into
a virtualenv.

Click is perfectly equipped to work with that and in fact the rest of the
documentation will assume that you are writing applications through
setuptools.

I strongly recommend to have a look at the :ref:`setuptools-integration`
chapter before reading the rest as the examples assume that you will
be using setuptools.
