I've referenced a video from Anthony Sotile in passing conversation several times. Walking through his gradual typing process has really helped me understand typing better, and has helped me make some projects better over time rather than getting slammed with typing errors.

Step 1

Run Mypy as is, don't get fancy yet. This will not reach into any functions unless they are alreay explicitly typed. It will not enforce you to type them either.

pip install mypy
mypy .
# or your specific project to avoid .venvs
mypy src
# or a single file
mypy my-script.py

Step 2

Next we will add check-untyped-defs, this will start checking inside functions that are not typed. To add this to your config create a setup.cfg with the following.

check_untyped_defs = True

Step 3

The final stage to this series is to add disallow_untyped_defs. This will start requiring all of your functions to be type hinted. This one is probably the toughest, because as you type functions mypy can uncover more issues for you to fix. Often times the list of errors grows before it shrinks.

check_untyped_defs = True
disallow_untyped_defs = True

Anthony's video

Make sure that you watch Anthony's video, give him a sub, he deserves it for all the great things he is doing for the python community.