Python Environment Markers

Enviroment markers are used to specify package versions based on the environment. This can include the platform (ie. win32 vs linux), the python interpreter version (ie. 2.7 vs 3.5). They are denoted by a ; after a requirement line (in a requirements.txt) and followed by the constraint.

For example:

requests [security,tests] >= 2.8.1, == 2.8.* ; python_version < "2.7"

This allows for only installing packages based on the defined condition.