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Intro to Static Type Checking


The most exciting thing happening right now in Python development is static typing. Since Python 3.0, we’ve had function annotations, and since 3.6, variable annotations. In 3.5, we got a “typing” library, which provides tools to describe types. This is what static type hints look like:

def f(x: int) -> int:
    return x * 5

This does nothing at runtime, except store the object. And in the upcoming Python 3.11 (or 3.7+ with from __future__ import annotations), it doesn’t even store the actual object, just the string you type here, so then anything that can pass the Python parser is allowed here.

It is not useless though! For one, it helps the reader. Knowing the types expected really gives you a much better idea of what is going on and what you can do and can’t do.

But the key goal is: static type checking! There are a collection of static type checkers, the most “official” and famous of which is MyPy. You can think of this as the “compiler” for compiled languages like C++; it checks to make sure you are not lying about the types. For example, passing in anything that is not an int to f will fail a mypy check, before you run or deploy any code.

Your tests cannot test every possible branch, every line of code. MyPy can (though it doesn’t by default, due to gradual typing). You may have code that runs rarely, that requires remote resources, that is slow, etc. All those can be checked by MyPy. It also keeps you (too?) truthful in your types.

Adding types

There are three ways to add types.

  1. They can be inline as annotations. Best for Python 3 code, usually.
  2. They can be in special “type comments”. Required mostly for Python 2 code, and still requires the proper imports.
  3. They can be in a separate file with the same name but with a .pyi extension. This is important for type stubs or for cases where you don’t want to add imports or touch the original code. You can annotate compiled files or libraries you don’t control this way.

If you have a library you don’t control, you can add “type stubs” for it, then give MyPy your stubs directory. MyPy will pull the types from your stubs. If you are writing code for a Raspberry Pi, for example, you could add the stubs for the Pi libraries, and then validate your code, without ever even installing the Pi-only libraries!

You do not have to add types for every object - most of the time, you just need it for parameters and returns from functions. When running MyPy, you can use reveal_type(...) to show the inferred type of any object, which is like a print statement but at type-checking time, or reveal_locals() to see all local types.


By default, MyPy does as little as possible, so that you can add it iteratively to a code base. By default:

  • All untyped variables and return values will be Any.
  • Code inside untyped functions is not checked at all.

You can add configuration to pyproject.toml (and a little bit to the files themselves), or you can go all the way and pass --strict, which will turn on everything. Try to turn on as much as possible, and increase it until you can run with full strict checking. See the style page for configuration suggestions.

For a library to support typing, it has to a) add types using any of the three methods, and b) add a py.typed empty file to indicate that it’s okay to look for types inside it. MyPy also looks in typeshed, which is a library full of type hints for (mostly) the standard library.

Third party libraries that are typed sometimes forget this last step, by the way!


Type narrowing

One of the key features of type checking is type narrowing. The type checker monitors the types of a variable, and “narrows” it when something restricts it. For example:

x: Union[A, B]
if isinstance(x, A):

This will print A, then B. It prints A because that’s the only thing that can exist in the first branch, and then the remaining types (B) must be the type in the second branch. And it prints both because it’s not actually running the code, just type checking it, so both sides of the if are checked.

You can manually force type narrowing with assert:

x: Union[A, B]
assert isinstance(x, A)

This will print A because you removed B via the type narrowing using the assert.


One of the best features of MyPy is support for structural subtyping via Protocols - formalized duck-typing, basically. This allows cross library interoperability, unlike traditional inheritance. Here’s how it works:

from typing import Protocol  # or typing_extensions for < 3.8

class Duck(Protocol):
    def quack() -> str:

Now any object that can “quack” (and return a string) is a Duck. We can even add @runtime_checkable which will allow us to check this (minus the types) at runtime in isinstance. So now we can design code like this:

def pester_duck(a_duck: Duck) -> None:

And the type checker will ensure we only write code valid on all Ducks. And, we can write a duck implementation and test it like this:

class MyDuck:
    def quack() -> str:
        return "quack"

This will pass a check for being a Duck, for example something like this:

import typing

if typing.TYPE_CHECKING:
    _: Duck = typing.cast(MyDuck, None)

Notice the complete lack of dependencies here. We don’t need MyDuck to write pester_duck, or vice-versa. And, we don’t even need Duck to write either one at runtime! The dependence on Duck for pester_duck is entirely a type-check-time dependence (unless we want to use a runtime_checkable powered isinstance).

There are lots of built-in Protocols, most of which pre-date typing and are available in an Abstract Base Class form. Most of them check for one or more special methods, like Iterable, Iterator, etc.

Other features

Static typing has some great features worth checking out:

  • Unions (New syntax coming in Python 3.10)
  • Generic Types (New syntax in Python 3.9)
  • Literals
  • TypedDict
  • Nicer NamedTuple definition (very popular in Python 3 code)
  • MyPy validates with the Python version you ask for, regardless of what version you are actually running.

Complete example

Python 3.6+

Here’s the classic syntax, which you need to use if support 3.6+.

from typing import Union, List

# Generic types take bracket arguments
def f(x: int) -> List[int]:
    return list(range(x))

# Unions are a list of types that all could be allowed
def g(x: Union[str, int]) -> None:
    # Type narrowing - Unions get narrowed
    if isinstance(x, str):
        print("string", x.lower())
        print("int", x)

    # Calling x.lower() is invalid here!

Python 3.7+

from __future__ import annotations

def f(x: int) -> list[int]:
    return list(range(x))

def g(x: str | int) -> None:
    if isinstance(x, str):
        print("string", x.lower())
        print("int", x)

Notice that there are no imports from typing! Note that you cannot use the “new” syntax in non annotation locations (like unions in isinstance) unless Python supports it at runtime.

You can use the above in earlier Python versions if you use strings.

Final words

When run alongside a good linter like flake8, this can catch a huge number of issues before tests or they are discovered in the wild! It also prompts better design, because you are thinking about how types work and interact. It’s also more readable, since if I give you code like this:

def compute(timestamp):

You don’t know “what” timestamp is. Is it an int? A float? An object? With types, you’ll know what I was intending to give you. You can use type aliases to really give expressive names here!