Is there a way to modify PYTHONPATH automatically whenever I cd into a directory.
I usually have multiple projects on my workstation, and whenever I am in one of those directories, I want that projects' src/ to override other src directories in PYTHONPATH.
Have you tried adding './src' to the PYTHONPATH ahead of the other paths that you want to "override"?  That ought to work.  (Haven't tried it with Python, but most "path" lists allow relative paths to be used.)
Modify PYTHONPATH automatically