ImportError: No module named error

[snippsat]

You get error because you run Python 2 /usr/bin/pythonw

Doing it Right Mac
Then you test,if follow that link python will point to python 3.

# Do I have a Python 3 installed?
$ python --version # or python3 --version
Python 3.6.5 # Success!
# If you still see 2.7 ensure in PATH /usr/local/bin/ takes pecedence over /usr/bin/

If python3 point to your python 3.6.5.

$ python3 WeatherDesk.py --help