Some potential gaps in the testing: it would be good to test the file format parsing, and exposing and testing _from_stream should enable this, as that part of the call would not touch the file system, environment variables, etc. Coverage of the error cases is important too.

There seems to be no coverage of the environment variable or well known file discovery paths, including errors. These are not straight-forward to test so, it might not be inconsistent with the depth of the testing that exists already to skip or defer these. In Java I did this with an internal class holding both the state and virtual methods to allow mocking things like checking for files or reading environment variables.