How to run a Python project using __pycache__ folder?

You can enforce the same layout of pyc-files in the folders as in Python2 by using:

python3 -m compileall -b test3

The option -b triggers the output of pyc-files to their legacy-locations (i.e. the same as in Python2).

After that you can once again use the compiled cache via:

python3 main.pyc

The way the loading of modules works since PEP-3147, it is impossible to use pyc-files from __pycache__ folder in the way you intend: If there is no *.py-file, the content of the __pycache__ is never looked-up. Here is the most important part of the workflow:

   import foo     |     |     -- >  [foo.py exists?]  --- NO ----> [foo.pyc exists?]  -- NO --> [ImportError]             |                                     |            |                                    YES           YES                                    |--> [load foo.pyc]            |            |-> [look up in __pycache__]                               

That means, files from __pycache__ are only looked up, when a corresponding *.py-file can be found.

Obviously, building python scripts with a Python version 3.X in this way and trying to run the resulting pyc-files with another Python version 3.Y will not work: Different Python versions need different pyc-files, this is the whole point behind PEP-3147.