As you mentioned, os.path.join can also be used for each individual path you have to append in the case you have to construct them from separate parts.
The caveat to be aware of with modifying environment variables in Python, is that there is no equivalent of the "export" shell command. There is no way of injecting changes into the current process, only child processes.
Please note that os.environ is not actually a dictionary. It's a special dictionary-like object which actually sets environment variables in the current process using setenv.
>>> os.environ.__class__
<class os._Environ at 0x100472050>
>>> import os
>>> os.environ["HELLO"] = "WORLD"
>>> os.getenv("HELLO")
'WORLD'
This means that PATH (and other environment variables) will be visible to C code run in the same process.