Python "importlib.resources" FileNotFoundError for Package Data

Reading a bundled data file with a filesystem path works from a source checkout but fails once installed, because the file may live inside a zip or simply was never packaged. importlib.resources is the supported way - but the data must actually ship in the wheel.

What this error means

Code that loads a template/config/data file raises FileNotFoundError in CI after an install (or in a zipapp/Lambda), while it works when run from the repo. The data file is missing from the installed package or read by a fragile relative path.

Python traceback

FileNotFoundError: [Errno 2] No such file or directory:
'/usr/lib/python3.11/site-packages/myapp/data/config.yaml'

Common causes

Data file not included in the wheel

Without declaring package data, the build excludes non-.py files. The import works but the data file is simply not present in the installed package.

Reading by filesystem path instead of resources

Using open(os.path.join(os.path.dirname(__file__), "data.yaml")) assumes an unpacked layout. In a zip import or relocated install, that path does not exist.

How to fix it

Include the data in the package

Declare package data so the files ship in the wheel.

pyproject.toml

[tool.setuptools.package-data]
myapp = ["data/*.yaml"]

Read it via importlib.resources

Use the resources API so it works whether the package is on disk or zipped.

Python

from importlib.resources import files

cfg = files("myapp.data").joinpath("config.yaml").read_text(encoding="utf-8")

How to prevent it

Declare every shipped data file as package data.
Access bundled files with importlib.resources, never raw paths.
Test against an installed wheel, not just the source tree.