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The Comprehensive Guide to mypy

Tushar Sadhwani on May 05, 2021

Read the original on my personal blog Mypy is a static type checker for Python. It acts as a linter, that allows you to write statical...

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Artalus • May 7 '21

I thought I use typehints a lot, but I have not yet encountered half of the things described here! Thank you for such an awesome and thorough article :3

Question. What do you think would be best approach on separating types for several concepts that share the same builtin type underneath? To avoid something like:

time_seconds: int = 1
time_ms: int = 1000
weight_kg = 61
def foo(start_ms: int, end_ms: int): ...
foo(time_seconds, weight_kg)

In modern C++ there is a concept of ratio heavily used in std::chrono to convert seconds in milliseconds and vice versa, and there are strict-typing libraries for various SI units. Would be nice to have some alternative for that in python.

Tushar Sadhwani • May 7 '21 • Edited

oh yea, that's the one thing that I omitted from the article because I couldn't think up a reason to use it. mypy has NewType which less you subtype any other type

like you can do ms = NewType('ms', int) and now if your function requires a ms it won't work with an int, you need to specifically do ms(1000). But in python code, it's still just an int. I think that's exactly what you need.

Artalus • May 8 '21

I think that's exactly what you need.

Totally! The ultimate syntactic sugar now would be an option to provide automatic "conversion constructors" for those custom types, like def __ms__(seconds: s): return ms(s*1000) - but that's not a big deal compared to ability to differentiate integral types semantically.

Thank you :)

Tushar Sadhwani • May 8 '21

Knowing that it's Python, I'm pretty sure that's easy to patch in on your side as well :)

Tushar Sadhwani • May 7 '21

I'm going to add NewType to the article now that I have a reason to :)

Benjamin Golinvaux • Apr 26 '22

Thanks for this very interesting article. Same as Artalus below, I use types a lot in all my recent Py modules, but I learned a lot of new tricks by reading this.

Hasanul Islam • Jul 1 '21 • Edited

Superb!
I am using pyproject.toml as a configuration file and stubs folder for my custom-types for third party packages.

Running from CLI, mypy . runs successfully. but when it runs at pre-commit, it fails (probably assuming stubs not present and thus return type is Any)

Corey Schaf • May 6 '21

Great post! I use type hinting all the time in python, it helps readability in larger projects.

Danny Kim • May 10 '21

In JavaScript ecosystem, some third-party libraries have no Typescript support at all or sometimes have incorrect types which can be a major hassle during development. How's the status of mypy in Python ecosystem?

Tushar Sadhwani • May 10 '21

Mypy is still fairly new, it was essentially unknown as early as 4 years ago. But the good thing about both of them is that you can add types to projects even if the original authors don't, using type stub files, and most common libraries have either type support or stubs available :)

Not much different than TypeScript honestly.