[python] How to fix "ImportError: No module named ..." error in Python?

What is the correct way to fix this ImportError error?

I have the following directory structure:

/home/bodacydo
/home/bodacydo/work
/home/bodacydo/work/project
/home/bodacydo/work/project/programs
/home/bodacydo/work/project/foo

And I am in the directory

/home/bodacydo/work/project

Now if I type

python ./programs/my_python_program.py

I instantly get

ImportError: No module named foo.tasks

The ./programs/my_python_program.py contains the following line:

from foo.tasks import my_function

I can't understand why python won't find ./foo/tasks.py - it's there.

If I do it from the Python shell, then it works:

python
>>> from foo.tasks import my_function

It only doesn't work if I call it via python ./programs/my_python_program.py script.

A better fix than setting PYTHONPATH is to use python -m module.path

This will correctly set sys.path[0] and is a more reliable way to execute modules.

I have a quick writeup about this problem, as other answerers have mentioned the reason for this is python path/to/file.py puts path/to on the beginning of the PYTHONPATH (sys.path).

In my mind I have to consider that the foo folder is a stand-alone library. I might want to consider moving it to the Lib\site-packages folder within a python installation. I might want to consider adding a foo.pth file there.

I know it's a library since the ./programs/my_python_program.py contains the following line:

from foo.tasks import my_function

So it doesn't matter that ./programs is a sibling folder to ./foo. It's the fact that my_python_program.py is run as a script like this:

python ./programs/my_python_program.py

Do you have a file called __init__.py in the foo directory? If not then python won't recognise foo as a python package.

See the section on packages in the python tutorial for more information.

Here is a step-by-step solution:

1. Add a script called run.py in /home/bodacydo/work/project and edit it like this:

   import programs.my_python_program
   programs.my_python_program.main()
   

   (replace main() with your equivalent method in my_python_program.)

2. Go to /home/bodacydo/work/project
3. Run run.py

Explanation: Since python appends to PYTHONPATH the path of the script from which it runs, running run.py will append /home/bodacydo/work/project. And voilà, import foo.tasks will be found.

If you have this problem when using an instaled version, when using setup.py, make sure your module is included inside packages

setup(name='Your program',
    version='0.7.0',
    description='Your desccription',
    packages=['foo', 'foo.bar'], # add `foo.bar` here

Example solution for adding the library to your PYTHONPATH.

1. Add the following line into your ~/.bashrc or just run it directly:

   export PYTHONPATH="$PYTHONPATH:$HOME/.python"
   

2. Then link your required library into your ~/.python folder, e.g.

   ln -s /home/user/work/project/foo ~/.python/