Just ran into this today, and wanted to get it down so I never forget:

Modules are not packages! 🙂

From the docs, a Module is:

A module is a file containing Python definitions and statements. The file name is the module name with the suffix .py appended. Within a module, the module’s name (as a string) is available as the value of the global variable __name__.

And a Package is:

Packages are a way of structuring Python’s module namespace by using “dotted module names”.

When you create a module and you want other scripts to be able to reference it, you need an __init__.py in the directory containing the modules in order for Python to treat a directory as containing a package.

The error manifests as “ImportError: No module named XXXX”.

All I had to do was touch __init__.py and things worked. I’m sure there are other things I’m going to learn today, but that was a silly one that I didn’t expect, and wasn’t mentioned in the Modules documentation, but is mentioned in Packages.