Reflecting on 2016, I began thinking about all of the tools I use on my computer for programming, productivity and the web. For the tools that we use everyday, we have lots of shortcuts and subtle habits here and there. I’m not always conscious of the many habits I have on my computer, but I want to start thinking about why and how I do things certain ways.

This post is the first in a series documenting the workflows and tools I use on a regular basis. Below is a half-baked list of posts in this series (the order and content may change):

  • mypy
  • vim and plugins
  • ack and jq
  • Todoist
  • Gmail
  • upstart
  • jekyll
  • to be continued..

Without further ado, let’s get started with the first on the list, mypy.


As I’ve learned and written more Python and Java, one of my biggest complaints about Python has been the lack of any sort of typing. Quite a few bugs I’ve created would’ve been discovered before run-time with Java, for example. There are other times when working in large code-bases it’s simply not clear what is being passed from method to method. This is where mypy can help.

Mypy is essentially a linting tool that statically type checks your Python code. As you write code, if you add type annotations to the classes, variables, and other elements, mypy is able to type check and find typed errors. There are a few different ways of adding annotations depending on Python version, but overall they are just hints in the code. Mypy is a static analyzer, and will never interfere with your code at run-time, it simply helps you find bugs and understand your code.

Installation and Setup

To install, mypy requires Python 3.3 or later to use. You can install it by running this:

python3 -m pip install mypy

After that, if you run which mypy and can see the install path, you’re all set to get started!

Examples and Basic Usage

Let’s look at a basic example of annotating a method annotation in Python 3. Here is an example function:

def say_hello(name: str) -> str:
    return "hi my friend %s" % name


There are two annotations here, one for the method argument and one for the return value. We can use mypy by running the following command: mypy You’ll see the following complaint from mypy: error: Argument 1 to "say_hello" has incompatible type "int"; expected "str"

While the above code still runs because of the duck-typing properties of Python, mypy loads the argument annotation and detects the mismatch. Not only do these annotations help catch bugs, but it makes it much easier to tell what the function actually does.

As a side-note, the syntax and usage for Python 2 is slightly different, so check out the documentation for Python 2.7.

You can also set the type of a variable with an inline notation, such as shown below:

class Car:
    speed = 0  # type: int
    direction_list = []  # type: List[int]

car = Car()
car.speed = 3  # this is valid
car.speed = "3"  # this is not

In the above, we annotate the types of two class variables using inline annotations. When accessing and mutating class variables, mypy applies type annotations to validate assignments as well.

Mypy comes with some built-in types, such as the str shown above. Here is a brief list of the most common ones:

  • int: integer of arbitrary size
  • float: floating point number
  • bool: boolean value
  • str: unicode string
  • bytes: 8-bit string
  • object: Python object, the common base class
  • List[str]: list of str objects
  • Dict[str, int]: dictionary hash from str to int values
  • Iterable[bytes]: iterable object of bytes
  • Sequence[float]: sequence of float objects
  • Any: dynamically typed object that can be of any type

You can use these types in method definitions, class variable annotations, and where else you would like to annotate types. As a note, to use the types List, Dict, Iterable, Sequence or Any, you need to import them from the typing module. If you have already installed mypy, this won’t be a problem, and here’s what an import line might look like for you:

from typing import Any, Dict, List

Contributing to Typeshed

While your own classes and methods can be annotated by yourself, it’s inevitable that you will use built-in Python classes and methods. To store these built-in annotations, mypy has a submodule called typeshed that you can contribute to. It stores the class and method definitions for Python’s built-in modules as well as some third-party packages.

While the majority of Python’s built-in classes are stubbed out, there are occasionally bugs and classes that don’t have stubs. Check out the issues for the latest bugs that they need help with over there.

In terms of workflow, it can be a little tricky to test out your typeshed changes without messing with your own mypy installation. Here’s a brief description of how I contribute to the project.

The setup is pretty simple, just clone the mypy repo to your local machine. Just make sure that this location is different from where you installed mypy previously. You’ll also want to fork the typeshed repo so you can set your fork as a remote to push to. Use git remote add <your_fork_name> <your_fork_location_url> to set it up.

After that, it’s pretty simple when it comes time to start working on an issue:

  • from the typeshed sub-directory, checkout a new branch to identify your issue - git checkout -b <branch_name>
  • make changes/fixes to typeshed stubs
  • once you make your changes, it’s time to run mypy using your changes, not the installed ones. The simplest way (as recommended by Guido himself) is to run mypy using a command like this:
PYTHONPATH=~/your/path/to/your/mypy_clone python3 -m mypy -f <flags>

In the above, you run mypy as a python package using your cloned mypy repo so that your changes are tested. Once you’re satisfied, changes can be pushed to your fork and you can open a pull-request from GitHub.

Learning More

This is a really brief introduction to mypy, and you’ll be able to find much more comprehensive explanations and documentation online. I highly recommend the mypy home page as it explains a lot of the motivations and reasoning for mypy. Just for documentation, the mypy documentation are incredibly helpful as well.

If you use mypy and have had good experiences with it, I would love to hear about it!