In this post, we'll cover the very basics of working with Python and
Click to create a CLI app. We'll install Python 3, set up our
development environment, install Click, and create a
Getting started with Python is easy. Most *nix systems come with Python
pre-installed. We'll be using Python 3. To check which version of
Python you have, open up your terminal and run
You should get an output similar to this:
This means the default Python version installed on my computer is
2.7.10. Most Macs ship with Python 2 by default. If you got
3.x.x, then you're good to go. After installing Python
python3 -V outputs:
If you don't have Python installed, you can head over to python.org, download the appropriate installer for your system, and run it. This step is necessary if you're on Windows.
pip is a package manager for Python. It allows us to
install libraries and dependencies that we'll need to use for our
project. It comes pre-installed with Python. To check if you have it,
you can run
pip -V or
pip3 -V. You should see
pip 19.0.3 from /usr/local/lib/python3.7/site-packages/pip (python 3.7)
When working on your Python projects, it's always a good idea to create
isolated environments to work in. This is because different projects
will require different dependencies or different versions of those
dependencies. Installing these globally will be hard to manage and will
just mess up your system. Installing them in a project's environment
keeps things clean. There's a couple of options to deal with
environments in Python but we'll focus on
We're going to create a new directory and create a virtual environment in it. On your terminal, run:
$ mkdir hello-world-cli && cd hello-world-cli
The command above creates a new directory called
hello-world-cli and enters that directory.
Next, we'll install
create our virtual environment.
$ pip3 install virtualenv
$ virtualenv venv
We now have a Python virtual environment in our directory. To use it, we need to activate it by running:
$ source venv/bin/activate (venv) $
You'll notice the change in your terminal prompt when the environment is activated. The environment name is prefixed at the prompt. That's how you know you're working in the environment.
Now that we have our environment ready, we can start working on our project. Python provides a number of packages to help in creating CLI apps. We'll be using Click, so we need to install it to get started.
(venv) $ pip install Click
That's all we need to get started.
As usual, we'll start off with a "Hello World!" program.
Still in the
hello-world-cli directory, create a file
(venv) $ touch hello-world.py
Open the file in your preferred text editor and add this code:
import click @click.command() def hello(): click.echo('Hello World!') if __name__ == '__main__': hello()
Let's examine what's happening at each line.
Here, we import
click so we can use it to create our
Whenever you see something on top of a function with an
symbol in Python, that's a decorator. Without going into a lot of
details on decorators, it's enough to understand (for now) that they
simply modify the behavior of the functions they "decorate". Click
uses the concept of decorators to convert Python functions into commands
that can be directly executed through the terminal. The decorator here
hello() into a command. We'll learn more about
This is how we define functions in Python.
hello() will run
click.echo('Hello World') which displays the text "Hello
World!" on the terminal.
if __name__ == '__main__':
This is the main entry point of our script.
This invokes our function/command.
Ok, time to see the results. Save the file, go back to the terminal, and run the program:
(venv) $ python hello-world.py Hello World!