How to pass .gitlab-ci.yml dependencies between steps

My .gitlab-ci.yml looks like this:

stages:
  - Linter

install:
  stage: Linter
  tags: ['checker']
  script:
    - pip install -r path/to/requirements.txt

black:
  stage: Linter
  tags: ['checker']
  needs: ['install']
  script:
    - python3 -m black --line-length 140 --check .

isort:
  stage: Linter
  tags: ['checker']
  needs: ['install']
  script:
    - python3 -m isort --check-only .

flake8:
  stage: Linter
  tags: ['checker']
  needs: ['install']
  script:
    - python3 -m flake8 .

Since the project is a Django app, I need some custom dependencies installed before performing black, isort and flake8 checks. The problem above is that apparently after install is done, the other jobs cannot detect installed dependencies. So all three get errors No module named black (or flake8, or isort).

I assumed that needs is what I need to add, but it does not change anything.

You have to split your jobs into two stages.
In the first stage you have to install the packages and after that you can pass the packages to the second stage (Linter) by using the artifacts keyword (see https://docs.gitlab.com/ee/ci/yaml/#artifacts).

You can use a virtual env to install your python packages. This might make it easier with the paths for the artifacts.