Python Type Annotations

How I Learned to Stop Worrying and Love Type Annotation

Gerard Keating
PyCon Ireland 2023

Brief History

  • 1990
    Guido invented a dynamically typed language called Python
  • 2014
    Guido wrote PEP 484 – Type Hints and type annotations were introduced in Python ~3.6
  • Today Python is still a dynamically typed language
var1: int = "hello"

# <class 'str'>

class Foo:

except AttributeError as err:
# AttributeError: type object 'Foo' has no attribute 'var1'
>>> class Foo:
...     var1:int
>>> Foo.__dict__
>>> mappingproxy(
              {'__module__': '__main__',
              '__annotations__': {'var1': int},
              '__dict__': <attribute '__dict__' of 'Foo' objects>,
              '__weakref__': <attribute '__weakref__' of 'Foo' objects>,
              '__doc__': None})

{'var1': int}
  • Type annotations does not really affect the runnning of the code
  • Useful as a form of structured documentation

Incorrect documentation is often worse than no documentation

Bertrand Meyer

Use a Type Checker

var1: int = "hello"

mypy error: Incompatible types in assignment (expression has type "str", variable has type "int") [assignment]

Type Checkers

Use mypy

  • Is the most mature and widely used
  • It catches the most errors
  • Alternatives:
    • Pytype -> Google
    • Pyright / Pylance -> Microsoft, vscode
    • Pyre -> Meta
    • Ruff -> Linter with some type checking
    • Others in your IDE

Other uses for Type Annotation

  • dataclass (and attrs): use it to to help define classes
  • pydantic: used for data validation
  • fastAPI: uses pydantic and uses annotations for documentation
  • SQLModel and other ORMs

Note on terminology

These refer to pretty much the same thing:

  • Type Annotation
  • Type Hinting
  • Type Decorations
  • Type Comments

For this presentation I'll be using the term Type Annotation

Note: Using Python 3.11

def new_way(bar:list[int])->int:
	return bar[5]

python 3.6 error:

Traceback (most recent call last):
  File "", line 1, in <module>
    def new_way(bar: list[int]) -> int:
TypeError: 'type' object is not subscriptable

Note: Some great features in Python 3.12

Type Annotation Types

int_: int = 1
float_: float = 1.5
string_: str = "hello"
bytes_: bytes = b"bytes"
none: None = None

class Foo:
my_foo: Foo = Foo()


class Animal:

class Dog(Animal):

lassie: Dog = Dog()
bluey: Animal = Dog()

garfield: Dog = Animal()
# error: Incompatible types in assignment
# (expression has type "Animal", variable has type "Dog")  [assignment]

Inheritance is a relationship.
A dog is a animal
An animal is not necessarily a dog

Type Annotation Syntax: Union |

var1: int|float|str = 6
  • var1 is of type int or float or string
  • Used to use typing.Union

Your own types

my_num = int|float
num: my_num = 6

Your type should have type annotation too

from typing import TypeAlias
my_text: TypeAlias = str|bytes
hello: my_text="Greetings"

Type Annotation Syntax: list

list1:list = [1]
list2:list[int] = [1]
list3:list[int|str] = [1, 'one']
list4:list[int|list[int]] = [1, [1]]
  • list1 is of type list
  • list2 is a list of ints
  • list3 is a list of ints or strings
  • list4 is a list of ints or lists of ints


  • Any means any type
  • Generally usage is a code smell
from typing import Any
list1: list
list2: list[Any]
  • list1: list is the same as list2: list[Any]

Type Annotation: dicts

dict1:dict = {"one":1}
# "dict" expects 2 type arguments (or no args)
dict2:dict[str, Any] = {"one":1}
dict3:dict[Any, int] = {"one":1}
dict4:dict[str, int|list[int|list[int]]] = {"one":1}
  • dict1 is the same as dict[Any, Any], any key type and any value type
  • dict2 is a dictionary with string type key and any value type
  • dict3 is a dictionary with string type key and any value type
  • dict4 you can work out yourself

Type Annotation: tuples

from typing import Any
simple: tuple = (1, 1,)
same_simple: tuple[Any, ...] = (1, 1, 's')
type_2int: tuple[int, int] = (1, 1,)
mix_types: tuple[int, str, float] = (1, 's', 0.5,)
all_ints: tuple[int, ...] = (1, 1, 1)
ints_strs: tuple[int | str, ...] = (1, 1, 's')
  • tuple accepts any number of type arguments
  • ... as second argument denotes any length

Type Annotation Syntax: Function

def bar(foo: list[int]) -> int:
    return bar[0]
  • bar takes a list of ints and returns an int

Type Annotation Syntax: classes

class Foo:
    bar: int # notice no value set
    val1: int | None = 7
    def __init__(self) -> None:
    def foo(self, arg1: int, arg2: str | int = 1) -> dict[str, int]:
        return {"one": 1}

  • Tip: if you want the type checker to check __init__ set its return type (to None)
  • self does not need to have its type annotated

Typing Literal

from typing import Literal

def open_helper(mode: Literal["r", "rb", "w", "wb"]) -> Literal[True]:

open_helper("x") # ❌

mypy will now check the argument matches:

error: Argument 1 to "open_helper" has incompatible type "Literal['x']"; expected "Literal['r', 'rb', 'w', 'wb']"  [arg-type]

Found 1 error in 1 file (checked 1 source file)

Note: this is not a runtime check

The Problem

def say_hello_to_joe(email_sender) -> None:

Solution -> Protocol

from typing import Protocol

class EmailSender(Protocol):
    def send_email(self,
            to: list[str],
            subject: str,
            text_body: str) -> None:

def say_hello_to_joe( email_sender: EmailSender) -> None:
        to=[email_address], subject="Hello", text_body="Hi")

class SendEmailRight:
    def send_email(self,
                to: list[str],
                subject: str,
                text_body: str) -> None:
        # actual implentation of sending email


The Problem

def first_item_bad(items: list[Any]) -> Any:
    return items[0]

first_item_bad(["hello"]) + 1
  • mypy will not complain
  • But there is a TypeError


T = TypeVar('T')

def first_item_better(items: list[T]) -> T:
    return items[0]

first_item_better(["hello"]) + 1
# ^ error: Unsupported operand types for + ("str" and "int")  [operator]
first_item_better([1]) + 1 # no error
  • Now mypy errors correctly

TypeVar adding constraints

TItem = TypeVar('TItem', str, int)

def first_item_best(items: list[TItem]) -> TItem:
    return items[0]

# ^ error: Value of type variable "TItem" of "first_item_best" cannot be "None"  [type-var]
# ^ error: Unsupported operand types for + ("str" and "int")  [operator]
  • mypy correctly errors on first_item_best([None]) and first_item_best(["hello"])+1


  • Final names cannot be reassigned in any scope
from typing import Final

DEBUG : Final[bool] = True

DEBUG = True # ❌ type check error

typing docs for more

Screenshot of

To set your Python package as typed include an empty file called py.typed

Alt text


  • For packages you cannot alter the code of you can create stubs
  • stubs already exist for many packages that are not typed

mypy will actually find some stubs for you error: Library stubs not installed for "six" [import] note: Hint: "python3 -m pip install types-six"

Example stub from stubs/Flask-Cors/flask_cors/core.pyi

def probably_regex(maybe_regex: str | Pattern[str]) -> bool: ...
def re_fix(reg: str) -> str: ...
def try_match_any(inst: str, patterns: Iterable[str | Pattern[str]]) -> bool: ...

Type Checking

  • Type Annotation is just documentation
  • Example, this code runs fine:
def foo()->dict[str:int]:

But the type annotation is wrong, check in mypy:

error: "dict" expects 2 type arguments, but 1 given  [type-arg]
error: Invalid type comment or annotation  [valid-type]

Running mypy

  • Install mypy using pip or poetry or whatever
  • Can be run on whole packages or single files and can be configured with exclude logic
mypy src

Misconceptions about mypy

  • mypy will not complain about missing type annotations unless configured to do so
  • mypy will not check the bodies of untyped functions unless configured to do so
def foo(bar=1):
  foobar: str = 'foobar'
  return bar + foobar
# note: By default the bodies of untyped
# functions are not checked,
# consider using --check-untyped-defs  [annotation-unchecked]

Getting started with type checking your code

  1. Install mypy
  2. Run mypy on your code
  3. Fix mypy errors
  4. Setup mypy configuration
  5. Add mypy to your CI so it fails builds if any mypy errors happen
  6. [Optionally] Sporadically manually run mypy strict rules and do some fixes

mypy config

By default it uses the file mypy.ini with a fallback to .mypy.ini, then pyproject.toml, then setup.cfg

mypy.ini file

exclude = migrations/
ignore_missing_imports = True

CI/CD example

  executor: python/default
    - checkout
    - python/install-packages:
        pkg-manager: poetry
    - run:
        command: |
          poetry run mypy --strict pretty_numbers

CI/CD example

mypy --strict

  • Misconception that --strict just checks if everything has a type annotation
  • It actually enables all optional error checking flags which can change over time
  • example flags:
  --warn-unused-configs, --disallow-any-generics,
  --disallow-subclassing-any --disallow-untyped-calls,
  --disallow-untyped-defs, --disallow-
  --check-untyped-defs, --disallow-untyped-decorators, --warn-redundant-
  casts, --warn-unused-ignores,
  --warn-return-any, --no-implicit-reexport,
  equality, --extra-checks

Why Type Annotation

  • Documentation of code that helps other developers making your code more readable
  • Enables IDE features that enables you to write code faster

Type checker finds and prevents BUGS


Contact me is the 1st mention but only covers function