This is what I use:

import os, sys
sys.path.append(os.path.join(os.path.dirname(__file__), "lib"))

The two below alternate possibilities apply to both Python versions 2 and 3. Choose the way you prefer. All use cases are covered.

Example 1

Module is being imported from a subdirectory (./bar/baz) of a directory of currently running module (/some/path/foo).

main script:      /some/path/foo/foo.py
module to import: /some/path/foo/bar/baz/mymodule.py

Add in foo.py

import sys, os
sys.path.append(os.path.join(sys.path[0], 'bar', 'baz'))
from mymodule import MyModule

Example 2

Module is being imported from a subdirectory (./bar/baz) of parent directory (/some/path/) of currently running module (/some/path/foo).

main script:      /some/path/foo/foo.py
module to import: /some/path/bar/baz/mymodule.py

Add in foo.py

import sys, os
sys.path.append(os.path.join(os.path.dirname(sys.path[0]), 'bar', 'baz'))
from mymodule import MyModule

Explanations

• sys.path[0] is /some/path/foo in both examples
• os.path.join('a','b','c') is more portable than 'a/b/c'
• os.path.dirname(mydir) is more portable than os.path.join(mydir,'..')

See also

Documentation about importing modules:

• in Python 2
• in Python 3

How python finds its modules

Strictly taken, a module is a single python file, while a package is a folder containing python files, accompanied by a (can be empty) file named __init__.py, to tell python it is a package to import modules from. In both cases, modules need their .py extension, but importing them is done without (see further below).

By default, Python looks for its modules and packages in $PYTHONPATH.

To find out what is included in $PYTHONPATH, run the following code in python (3):

import sys
print(sys.path)

How to add a directory

Occasionally

From within a python file, you can add path(s) occasionally to the default path by adding the following lines in the head section of your python application or script:

import sys
sys.path.insert(0, "/path/to/your/package_or_module")

For example:

if I have a folder: /home/myname/pythonfiles, and I want to import the file module_1.py, located in that directory, I add this to the head section of my code:

import sys
sys.path.insert(0, "/home/myname/pythonfiles")

And I can simply import the file module_1.py by:

import module_1

When I create a package and want to import module(s) from the package, I need to create a folder in $PYTHONPATH, containing the modules, accompanied by a (can be empty) file called __init__.py

For example:

To import from a package (folder) called my_package in /home/myname/pythonfiles , add the /home/myname/pythonfiles path to your $PYTHONPATH, like in example 1, and import the module called module_2.py (inside the package folder) simply with: `

from <packagename> import module_2

Adding directories to $PYTHONPATH permanently:

Add the following line to your ~/.profile file.

export PYTHONPATH=$PYTHONPATH:/path/you/want/to/add

Subdirectories

From within a package, subdirectories are not included just like that; you need to "chain" the directories. To import a module module_3.py, inside folder subfolder inside folder packagename:

import packagename.subfolder.module_3

Given the fact that all subfolders in the package include their own __init__.py file.

When a module is in the same directory as the script or application

There is no need to insert the path to a module when it is in the same directory as the script or application, it is automatically added.

Example:

If I have a folder, containing script.py and module.py, I can simply import the module by:

import module

EDIT Nov 2014 (3 years later):

Python 2.6 and 3.x supports proper relative imports, where you can avoid doing anything hacky. With this method, you know you are getting a relative import rather than an absolute import. The '..' means, go to the directory above me:

from ..Common import Common

As a caveat, this will only work if you run your python as a module, from outside of the package. For example:

python -m Proj

Original hacky way

This method is still commonly used in some situations, where you aren't actually ever 'installing' your package. For example, it's popular with Django users.

You can add Common/ to your sys.path (the list of paths python looks at to import things):

import sys, os
sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'Common'))
import Common

os.path.dirname(__file__) just gives you the directory that your current python file is in, and then we navigate to 'Common/' the directory and import 'Common' the module.