Absolute imports in python not working, relative imports work

I can't seem to get absolute imports to work in python. Here is my file structure:

a/
  b/
    __init__.py
  __init__.py
  foo.py

foo.py:

# Including or removing the __future__ import has no effect
from __future__ import absolute_import
from a import b  # just `import b` works fine
print "success!"

I instead get:

ImportError: No module named a

What in the world is going on here? I feel like I'm missing something basic. While this example is trivial, I need absolute imports to work for the real project, so just replacing the import with a relative one is not an option. I'm using python 2.7.13

None of the solutions listed here help

Thanks in advance for any insight!