Python Environment Markers
Enviroment markers are used to specify package versions based on the environment. This can include the platform (ie.
linux), the python interpreter version (ie.
3.5). They are denoted by a
; after a requirement line (in a
requirements.txt) and followed by the constraint.
requests [security,tests] >= 2.8.1, == 2.8.* ; python_version < "2.7"
This allows for only installing packages based on the defined condition.