There are Environment Markers or Requirement Specifiers we can use in requirements.txt. For example, argparse;python_version<"2.7" will tell pip to install argparse only when the python version is less than 2.7.

However, is it possible to generate these environment markers when using pip freeze > requirements.txt? Documentation of pip freeze does not mention a way. I tried the -r option but had no luck. Do we have to edit the requirements.txt file manually after pip freeze?