The function columnflow.calibration.cms.jets.get_jerc_file_default returns str:

This function is the default for jec.get_jec_file and jer.get_jer_file.

Further down the code, .load() is called on the result of the get_*_file functions, e.g. for jec:

In my recent experience the type hint was right, meaning the defaults do not work 🤔