Should I use `import os.path` or `import os`?

os.path works in a funny way. It looks like os should be a package with a submodule path, but in reality os is a normal module that does magic with sys.modules to inject os.path. Here’s what happens:

• When Python starts up, it loads a bunch of modules into sys.modules. They aren’t bound to any names in your script, but you can access the already-created modules when you import them in some way.

  • sys.modules is a dict in which modules are cached. When you import a module, if it already has been imported somewhere, it gets the instance stored in sys.modules.

• os is among the modules that are loaded when Python starts up. It assigns its path attribute to an os-specific path module.

• It injects sys.modules['os.path'] = path so that you’re able to do “import os.path” as though it was a submodule.

I tend to think of os.path as a module I want to use rather than a thing in the os module, so even though it’s not really a submodule of a package called os, I import it sort of like it is one and I always do import os.path. This is consistent with how os.path is documented.

Incidentally, this sort of structure leads to a lot of Python programmers’ early confusion about modules and packages and code organization, I think. This is really for two reasons

1. If you think of os as a package and know that you can do import os and have access to the submodule os.path, you may be surprised later when you can’t do import twisted and automatically access twisted.spread without importing it.

2. It is confusing that os.name is a normal thing, a string, and os.path is a module. I always structure my packages with empty __init__.py files so that at the same level I always have one type of thing: a module/package or other stuff. Several big Python projects take this approach, which tends to make more structured code.