Python ImportError: No module named …

Symptom:

from MY_FOLDER.MY_FILE import MY_CLASS causes an error:
ImportError: No module named MY_FOLDER.MY_FILE

Debugging:

1. Execute the program in debug mode,
2. In the Variables window expand Special Variables,
3. Read the __warningregistry__ value.
   

Possible cause:

The __init__.py file is missing in the directory of the referenced class.

Solution:

Create an empty __init__.py file in the directory where the class is referenced from.