Mypy Primer
What is mypy, and how can I use it?
What is mypy?
Introduction to Python's type annotation syntax
Installing mypy on your machine, and in your code editor
Using mypy to type-check your code
We'll see what kind of errors mypy will throw at you, how to configure mypy to make the best use of it, and try to figure out what its errors mean.
Why doesn't mypy show an error here?
Mypy configuration
Simple type checking examples
Using reveal_type() to debug types
Modern Python has a LOT of type checking features baked into it. In this chapter we will see a glimpse of what the typing module has to offer.
Lists, Dictionaries, and other collections
Union and Optional types
Reading mypy error messages
Type checking your classes
Typing *args and **kwargs
The "Any" type
The typing module also provides various functions and base classes, that create the basis of a lot of concepts of mypy's type system. In this chapter we will dive deep into these functionalities.
Static duck typing
Custom Generic types
The NamedTuple class
The TypedDict class
Function overloading with @overload
In this chapter we will learn some concepts, tips and tricks on how to properly type-check code using some of the more complicated Python concepts.
Type checking decorators
Type checking generators
The Type type
In this chapter, we will wrap up the series, and understand where type checking sits in the ecosystem of Python, and your software development process.
Type checking, or writing tests? Why not both!
Integrating mypy into your software development lifecycle
All Courses Mypy Primer Installing mypy on your machine, and in your code editor
Installing mypy on your machine, and in your code editor
Tushar Sadhwani
Nov 1, 2021

Before we move ahead with the rest of the course, I want to explain how you can install mypy on your machine and code editor.

Installing mypy locally

To run mypy locally, you need to have Python installed.

  • If you're running Windows 10, you can open Command Prompt, and type python, which should either start Python, or open up Windows Store, where you can install it.
  • On a Mac or linux machine, you probably already have Python installed, under the name python3. Simply open a terminal and type python3 to confirm.

Now to install mypy, you need to run pip, a package manager which comes with Python:

$ python -m pip install mypy
Collecting mypy
[...]
Successfully installed mypy-1.x typing-extensions-4.x

Now you should have access to the mypy command:

$ mypy
usage: mypy [-h] [-v] [-V] [more options; see below]
            [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
mypy: error: Missing target module, package, files, or command.

To test it, pass it the name of any Python file:

$ mypy code.py
Success: no issues found in 1 source file

Editor integration

While mypy and the other type checkers can be used directly through the command line, it is a bit more convenient for IDE users if the type checking errors integrate directly with their editor. There are two popular editors for Python users which can do this, Visual Studio Code and PyCharm. Let’s take a look at them.

Visual Studio Code

VSCode has a Python extension that comes with an official language server called “Pylance”, which provides type information out of the box.

You can however, enable mypy support in VSCode as well, by adding the following to your settings.json file:

  ...
  'python.linting.mypyEnabled': true,
  'python.linting.mypyArgs': [
    '--ignore-missing-imports',
    '--follow-imports=silent',
    '--show-column-numbers',
    '--strict'
  ],
...

You should then be able to see type checking issues directly in your code:

If you don’t, restart your editor and that should resolve any issues.

PyCharm

PyCharm has created its own type inference and refactoring system that works quite well and provides things like attribute auto-completion in the IDE by default. If you want to integrate mypy however, a Plugin called “Mypy” made by Roberto Leinardi exists in the JetBrains Marketplace. Simply installing it should enable mypy checks for you:

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