Requirements & Dependencies
Home Assistant allows components and platforms to specify their dependencies and requirements using the variables DEPENDENCIES and REQUIREMENTS. Both are lists that contain strings.
Dependencies
Dependencies are other Home Assistant components that should be setup before the platform is loaded. An example is the MQTT sensor component, which requires an active connection to an MQTT broker. If Home Assistant is unable to load and setup the MQTT component, it will not setup the MQTT sensor component.
DEPENDENCIES = ['mqtt']
Requirements
Requirements are Python libraries or modules that you would normally install using pip for your component. Home Assistant will try to install the requirements into the deps subdirectory of the Home Assistant configuration directory if you are not using a venv or in something like path/to/venv/lib/python3.6/site-packages if you running in a virtual environment. This will make sure that all requirements are present at startup. If steps fails like missing packages for the compilation of a module or other install errors, the component will fail to load.
Requirements is a list of strings. Each entry is a pip compatible string. For example, the media player Cast platform depends on the Python package PyChromecast v0.6.12:
REQUIREMENTS = ['pychromecast==0.6.12']
During development of a component, it can be useful to test against different versions of a requirement. This can be done in two steps, using pychromecast as an example:
pip install pychromecast==0.6.13 --target ~/.homeassistant/depshass --skip-pip
This will use the specified version, and prevent Home Assistant from trying to override it with what is currently in REQUIREMENTS.
If you need to make changes to a requirement to support your component, it’s also possible to pip install from a checkout of the requirement.
git clone https://github.com/balloob/pychromecast.gitpip install ./pychromecasthass --skip-pip