The syntax that you are using is:

pip install "project[extra]"

In your case, you are installing the splinter package which has the added support for django. The square brackets ([]) are not specific syntax, just convention. Really, you are installing the package named: "splinter[django]".

An explanation from @chetner:

The command pip install splinter django would install two packages named splinter and django. splinter[django], on the other hand, installs a variant of the splinter package which contains support for django. Note that it has nothing to do with the django package itself, but is just a string defined by the splinter package for a particular feature set that gets enabled.

Brackets [optional] in PIP signify optional dependencies

Just in case another developer comes along looking to implement this pattern in their own Python package deployment, here's further explanation of the brackets [] in pip.

For Example: Apache Airflow

To install airflow from pip we use this command:

pip install 'apache-airflow'

You can install optional components of airflow with:

pip install 'apache-airflow[aws]'
#      [optional] -----------^

When we search pypi for apache-airflow note that the optional packages do not show up:

pip search 'apache-airflow'

apache-airflow (1.10.9)                            - Programmatically author, schedule and monitor data pipelines
pylint-airflow (0.1.0a1)                           - A Pylint plugin to lint Apache Airflow code.
swe-airflow-tools (0.0.3)                          - Tools for Apache Airflow Application
airflow (0.6)                                      - Placeholder for the old Airflow package
...

Implementation via setup.py

You can see how this was accomplished in the setup.py script
On the left in setup.py - extras_require is defined.
On the right are the correlated installation commands for these optional sub-packages.

Pretty sure these are setuptools extras:

https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies

Sometimes a project has “recommended” dependencies, that are not required for all uses of the project. For example, a project might offer optional PDF output if ReportLab is installed, and reStructuredText support if docutils is installed. These optional features are called “extras” ...