I want to get the absolute path of the brother directory in the directory where the executable is located.

From Comments

It's solved.We actually used os.path.dirname to specify the upper hierarchy.

_init__.py must be placed in order to import anything below the b folder.
_init__.py should be empty.
You can specify where this file is located in import.

Folder Configuration

a/
├ ├ hoge.py
└ b/
  ├ __init__.py
  └ └ c.py

hoge.py

 from b.import something

something()

c.py

def something():
    print('test')

By the way, the import method can also be import b.c.
However, if you import it that way, it must be b.c.something() when calling the method.

Class definitions can also be invoked.

hoge.py

 from b.import something, TestClass

something()

test_class = TestClass()
test_class.something()

c.py

def something():
    print('test')


class TestClass():
    def__init__(self):
        pass

    def something (self):
        print('class test')

Check the documentation for details.
http://docs.python.jp/2/tutorial/modules.html#packages