importing an attribute

[Gribouillis]

what Python "mechanism" is involved that makes the attribute unavailable until it is imported.

This is what happens when 'session' is a submodule of a package 'botocore'. You only need a file structure like this

Output:
botocore/
    __init__.py
    session.py

If there is no import session in __init__.py, then module botocore has no 'session' attribute but one can import botocore.session. You can also have a subpackage of course

Output:
botocore/
    __init__.py
    session/
        __init__.py