mypy

This lesson is part of the Automating Python Code Quality course.

In this lesson, you will learn:

what is mypy
how to install mypy
how to validate your type hints using mypy
mypy's main configuration options.

You can find the scripts and code for this course within the Automating Python Code Quality repo.

What is mypy?

Mypy is a static type checker for Python. It parses your type hints/annotations to find any potential issues and errors within your code. Mypy can be thought of as more of a linter, as it only reports the issues and no changes are made to your codebase.

Installing mypy

To install mypy we simply perform a poetry add -D like so:

$ poetry add -D mypy

Usage

Mypy can be run against a file or directory. For example:

$ mypy .
$ mypy dir
$ mypy myfile.py

Therefore, following on from the example in the previous lesson, let’s say we have the following file:

$ cat 003_type_checking/001_mypy_issues.py
vlan_id: int = "100"

def add_vlan(vlan_id: int):
    return vlan_id + 1

add_vlan(vlan_id)

We can now run mypy on our file (shown below). As a result, we will get the expected issue that the variable we have assigned to vlan_id is of the wrong type.

❯ mypy 003_type_checking/001_mypy_issues.py
003_type_checking/001_mypy_issues.py:1: error: Incompatible types in assignment (expression has type "str", variable has type "int")
Found 1 error in 1 file (checked 1 source file)