Q:

relative imports between directories from the same package

I'm having some trouble with relative imports for the following situation.
I have a package, with two module directories, and I want to import a module from dir_b to a module from dir_a.
Here's an example of my package structure:
$ tree
.
├── builder
│   ├── build_moto.py
│   └── __init__.py
├── __init__.py
└── parts
    ├── car.py
    ├── __init__.py
    ├── moto.py
    └── truck.py

I'm trying to import moto inside build_moto using relative imports, like this:
$ cat builder/build_moto.py 
#!/usr/bin/python3

from .parts import moto

...but when I execute build_moto.py, it generates the following error:
$ python3 builder/build_moto.py 
Traceback (most recent call last):
  File "builder/build_moto.py", line 3, in <module>
    from .parts import moto
SystemError: Parent module '' not loaded, cannot perform relative import

I'd like to understand: 

why this configuration is not working?
what must be done in order to perform a relative import for this case in specific?

A:

Often, this problem can be solved like this:
python3 -m builder.build_moto

The -m argument means that you are running your module as part of a library:

-m mod : run library module as a script (terminates option list) 

