How to run pylint only on changed files in Gitlab?

I’m trying to run pylint only on changed python files, but my build keeps failing. I have extracted the changed files through git diff and saved them in a variable, but when I inject the variable into the pylint call, it fails. It works fine with a hardcoded filename however. Here is my yaml:

pylint:
stage: test
  before_script:
    - pip install pylint pylint-exit anybadge
  script:
      - echo CI_COMMIT_SHA=${CI_COMMIT_SHA}
      - echo CI_MERGE_REQUEST_TARGET_BRANCH_NAME=${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}
      - git fetch origin ${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}
      - FILES=$(git diff --name-only ${CI_COMMIT_SHA} origin/${CI_MERGE_REQUEST_TARGET_BRANCH_NAME} | grep '.py'$)
      - echo "Changed files are $FILES"
      - pylint --output-format=text $(find -type f -name "$FILES" ! -path "**/.venv/**") | tee ./pylint/pylint.log || pylint-exit $?
      - PYLINT_SCORE=$(sed -n 's/^Your code has been rated at ([-0-9.]*)/.*/1/p' ./pylint/pylint.log)
      - anybadge --label=Pylint --file=pylint/pylint.svg --value=$PYLINT_SCORE 2=red 4=orange 8=yellow 10=green
      - echo "Pylint score is $PYLINT_SCORE"
  artifacts:
    paths:
      - ./pylint/
    when: always
  only:
      refs:
          - merge_requests
      changes:
          - "**/*.py"

Answer

While qathulu answer is adequate, it does not work if no python files are changed. The below prints all the changed files, then use grep to find the changed python files.

Unfortunately there is an error on gitlab, where assigning empty output from grep ends the pipeline. In order to fix that, grep is not assigned to variable until we are sure at least one python file is changed.

- echo CI_COMMIT_SHA=${CI_COMMIT_SHA}
- echo CI_MERGE_REQUEST_TARGET_BRANCH_NAME=${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}
- git fetch origin ${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}
- tmp_files=$(git diff --name-only ${CI_COMMIT_SHA} origin/${CI_MERGE_REQUEST_TARGET_BRANCH_NAME})
- echo "Changed files are $tmp_files"
- if [ -z "$(echo "$tmp_files" | grep ".py")" ]; then exit 0; else echo "Python files are found"; fi
- tmp_pfiles=$(echo "$tmp_files" | grep ".py")
- echo "Python files are $tmp_pfiles"
- mkdir ./pylint
- pylint --output-format=text $tmp_pfiles | tee ./pylint/pylint.log || pylint-exit $?