How it works...

When source code (.py) is read by the Python interpreter, the bytecode is generated and stored in __pycache__ as <module_name>.<version>.pyc. The .pyc extension indicates that it is compiled Python code. This naming convention is what allows different versions of Python code to exist simultaneously on the system.

When source code is modified, Python will automatically check the date with the compiled version in cache and, if it's out of date, will automatically recompile the bytecode. However, a module that is loaded directly from the command line will not be stored in __pycache__ and is recompiled every time. In addition, if there is no source module, the cache can't be checked, that is, a bytecode-only package won't have a cache associated with it.