The CMSSW environment can be extended to act as a Python virtual environment.
Assuming you are already in your CMSSW working area and have called cmsenv, execute the following commands:
scram-venv
cmsenv
Subsequently, when returning to the same CMSSW working area, just calling cmsenv will automatically activate the virtual environment.
Once the virtual environment is set up, any pip3 commands will automatically use the directories $CMSSW_BASE/venv/$SCRAM_ARCH/[bin,include,lib].
The implementation of Python virtual environments in CMSSW is based on the modern Python3 specification given in PEP 405.
It relies on the use of include-system-site-packages in pyvenv.cfg to identify Python packages distributed as part of the base CMSSW release (on cvmfs) as read-only.
This is accomplished via the use of sitecustomize.py when Python3 is built for CMSSW.
(For more details, see the site module documentation.)
scram-venv works with any CMSSW version that includes Python3.
Full compatibility starts with CMSSW_10_2_X.
CMSSW_10_1_X instructionsCMSSW_10_1_X has Python3 but not pip3, so some extra steps are required:
python3 -m ensurepip
python3 -m pip install --upgrade pip
Subsequently, the command python3 -m pip should be used rather than pip3.
CRAB support is currently under investigation. Please check back later!