Why doesn't os.path.join() work in this case?

The latter strings shouldn't start with a slash. If they start with a slash, then they're considered an "absolute path" and everything before them is discarded.

Quoting the Python docs for os.path.join:

If a component is an absolute path, all previous components are thrown away and joining continues from the absolute path component.

Note on Windows, the behaviour in relation to drive letters, which seems to have changed compared to earlier Python versions:

On Windows, the drive letter is not reset when an absolute path component (e.g., r'\foo') is encountered. If a component contains a drive letter, all previous components are thrown away and the drive letter is reset. Note that since there is a current directory for each drive, os.path.join("c:", "foo") represents a path relative to the current directory on drive C: (c:foo), not c:\foo.

The idea of os.path.join() is to make your program cross-platform (linux/windows/etc).

Even one slash ruins it.

So it only makes sense when being used with some kind of a reference point like os.environ['HOME'] or os.path.dirname(__file__).

os.path.join() can be used in conjunction with os.path.sep to create an absolute rather than relative path.

os.path.join(os.path.sep, 'home','build','test','sandboxes',todaystr,'new_sandbox')