How do I reference a file relatively to a package's directory?

My directory structure is:

    /foo
     package1/
      resources/
      __init__.py
     package2/
      resources/
      __init__.py
     script.py

script.py imports packages package1 and package2. Although the packages can be imported by any other script on the system. How should I reference resources inside, say, package1 to ensure it would work in case os.path.curdir is arbitrary?

python reference package relative-path
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