A minimal example of cached python environment in GitLab CI

.gitlab-ci.yml

##############################################################################
# Copyright (c) 2022, Lawrence Livermore National Security, LLC and RADIUSS
# project contributors. See the COPYRIGHT file for details.
#
# SPDX-License-Identifier: (MIT)
##############################################################################

variables:
  PYTHON_BIN_DIR: /usr/tce/packages/python/python-3.8.2/bin
  PYTHON_ENVIRONMENT_PATH: ${CI_PROJECT_DIR}/.venv
  PIP_CACHE_DIR: ${CI_PROJECT_DIR}/.cache/pip

stages:
  - env
  - run

.on-ruby:
  tags:
    - shell
    - ruby

# This jobs creates a python env and caches it.
# Note: The cache is updated automatically when the requirements.txt changes.
# Cache can be manually cleared by clicking "CI/CD - Pipelines - Clear Runner Caches".
configure_python:
  extends: [.on-ruby]
  stage: env
  script:
    - ${PYTHON_BIN_DIR}/virtualenv -p ${PYTHON_BIN_DIR}/python3 ${PYTHON_ENVIRONMENT_PATH}
    - . ${PYTHON_ENVIRONMENT_PATH}/bin/activate
    - pip install -r ./requirements.txt
  cache:
    key:
      files:
        - requirements.txt
    paths:
      - ${PYTHON_ENVIRONMENT_PATH}
      - ${PIP_CACHE_DIR}

.venv-readonly:
  cache:
    key:
      files:
        - requirements.txt
    paths:
      - ${PYTHON_ENVIRONMENT_PATH}
      - ${PIP_CACHE_DIR}
    policy: pull

job-using-cache-readonly:
  extends: [.venv-readonly, .on-ruby]
  stage: run
  script:
    - which python3
    - python3 --version
    - pip freeze
    - . ${PYTHON_ENVIRONMENT_PATH}/bin/activate
    - which python3
    - python3 --version
    - pip freeze


#requirements.txt (example)
# ###### Requirements without Version Specifiers ######
# lxml
#
# ###### Requirements with Version Specifiers ######
# # foo == 0.6.1   # Version Matching. Must be version 0.6.1
# # bar >= 4.1.1   # Minimum version 4.1.1
# # xxx != 3.5     # Version Exclusion. Anything except version 3.5
# # yyy ~= 1.1     # Compatible release. Same as >= 1.1, == 1.*