Python tip N5 : Gradual typing in python

Steps

Just run

mypy your-app

And see what you get.

Add this to your mypy config to ignore 3rd library imports that are not typed

disable_error_code = import-untyped

Run mypy, see what you get, and fix errors you might get

If you have some modules that are untyped, you can install them with

mypy --install-types

or with

pip install types-modulename

Add those after that

strict_equality = True
strict_concatenate = True

Try after that to make mypy check the functions that are untyped by adding this config (it is false by default)

check_untyped_defs = True

As you go by, you can then force having types in your codebase by adding the following

disallow_untyped_defs = True

Tools that can help

You can use some tools to automatically generate types for some parts of your code. Some tools that I found useful are https://monkeytype.readthedocs.io/en/latest/index.html, https://github.com/JelleZijlstra/autotyping

• You can run monkeytype on your tests, it will infer the types from your tests and store them in a sqllite db, you can then apply those types to your src folder
• You can use autotyping to add return types, or arg types in your code
• mypy-baseline is a tool that you can run on your codebase, and it will keep the state of errors, and only warn about new ones
• mypy-silent is also a tool that you can use to add type-ignore everywhere ..