import a.b also imports a?

Question

When I import a subpackage in a package, can I rely on the fact that the parent package is also imported ?

e.g. this works

python -c "import os.path; print os.getcwd()"

Shouldn't I explicitly import os for os.getcwd to be available ?

Answer 1

There's an important thing to know about packages, that is that there is a difference between being loaded and being available.

With import a you load module a (which can be a package) and make it available under the name a.

With from a import b you load module a (which definitely is a package), then load module a.b and make only this available under the name b. Notice that a also got loaded in the process, so any initialization this is supposed to do will have happened.

With import a.b you load both and make both available (under the names a and a.b).

Answer 2

This is a good question. If you look at the source code for os.py you find this line:

sys.modules['os.path'] = path

So there's our module. But what's path? Well that depends on your OS. For Windows, it's defined in this block:

elif 'nt' in _names:
    name = 'nt'
    linesep = '\r\n'
    from nt import *
    try:
        from nt import _exit
    except ImportError:
        pass
    import ntpath as path

    import nt
    __all__.extend(_get_exports_list(nt))
    del nt

Basically, os.path is special in this context.

Short version: Python does some stuff behind the scenes to make os.path. You probably shouldn't rely on it to get other modules. "Explicit is better than implicit" is how the zen goes.