Python module import - why are components only available when explicitly imported?

Question

I have recently installed scikit-image version 0.11.3. I am using python 2.7.10. When I import the entire module I cannot access the io module.

import skimage img = skimage.io.imread(path_) 

Gives error:

AttributeError: 'module' object has no attribute 'io' 

However the following does not error.

from skimage import io img = io.imread(path_) 

Question: Why?

Answer 1

Quick answer: IO is a submodule. Submodules need to be imported from the parent module explicitly.

Long answer: From section 5.4.2 of the python docs:

When a submodule is loaded using any mechanism (e.g. importlib APIs, the import or import-from statements, or built-in import()) a binding is placed in the parent module’s namespace to the submodule object. For example, if package spam has a submodule foo, after importing spam.foo, spam will have an attribute foo which is bound to the submodule. Let’s say you have the following directory structure:

spam/     __init__.py     foo.py     bar.py 

and spam/init.py has the following lines in it:

from .foo import Foo from .bar import Bar 

then executing the following puts a name binding to foo and bar in the spam module:

>>> >>> import spam >>> spam.foo <module 'spam.foo' from '/tmp/imports/spam/foo.py'> >>> spam.bar <module 'spam.bar' from '/tmp/imports/spam/bar.py'> 

Given Python’s familiar name binding rules this might seem surprising, but it’s actually a fundamental feature of the import system. The invariant holding is that if you have sys.modules['spam'] and sys.modules['spam.foo'] (as you would after the above import), the latter must appear as the foo attribute of the former.