How to cache Python module in Gitlab CI

By default, pip cache will be in ~/.pip. However, this folder cannot be cached by Gitlab. The trick is to force pip to use a folder located in the build directory with the --cache-dir option. You can then cache this folder.

For instance, you can use .pip as in the .gitlab-ci.yml excerpt below:

image: python:3.6

cache:
  paths:
    - .pip

before_script:
  - mkdir -p .pip
  - pip install -U pip
  - pip --cache-dir=.pip install -r requires.txt
  - pip --cache-dir=.pip install -r tests_requires.txt