attrs/docs/types.md

Type Annotations

attrs comes with first-class support for type annotations for both {pep}526 and legacy syntax.

However, they will remain optional forever, therefore the example from the README could also be written as:

>>> from attrs import define, field

>>> @define
... class SomeClass:
...     a_number = field(default=42)
...     list_of_numbers = field(factory=list)

>>> sc = SomeClass(1, [1, 2, 3])
>>> sc
SomeClass(a_number=1, list_of_numbers=[1, 2, 3])

You can choose freely between the approaches, but please remember that if you choose to use type annotations, you must annotate all attributes!

:::{caution} If you define a class with a {func}attrs.field that lacks a type annotation, attrs will ignore other fields that have a type annotation, but are not defined using {func}attrs.field:

>>> @define
... class SomeClass:
...     a_number = field(default=42)
...     another_number: int = 23
>>> SomeClass()
SomeClass(a_number=42)

:::

Even when going all-in on type annotations, you will need {func}attrs.field for some advanced features, though.

One of those features are the decorator-based features like defaults. It's important to remember that attrs doesn't do any magic behind your back. All the decorators are implemented using an object that is returned by the call to {func}attrs.field.

Attributes that only carry a class annotation do not have that object so trying to call a method on it will inevitably fail.

---

Please note that types -- regardless how added -- are only metadata that can be queried from the class and they aren't used for anything out of the box!

Because Python does not allow references to a class object before the class is defined, types may be defined as string literals, so-called forward references ({pep}526). You can enable this automatically for a whole module by using from __future__ import annotations ({pep}563). In this case attrs simply puts these string literals into the type attributes. If you need to resolve these to real types, you can call {func}attrs.resolve_types which will update the attribute in place.

In practice though, types show their biggest usefulness in combination with tools like Mypy, pytype, or Pyright that have dedicated support for attrs classes.

The addition of static types is certainly one of the most exciting features in the Python ecosystem and helps you write correct and verified self-documenting code.

Mypy

While having a nice syntax for type metadata is great, it's even greater that Mypy ships with a dedicated attrs plugin which allows you to statically check your code.

Imagine you add another line that tries to instantiate the defined class using SomeClass("23"). Mypy will catch that error for you:

$ mypy t.py
t.py:12: error: Argument 1 to "SomeClass" has incompatible type "str"; expected "int"

This happens without running your code!

And it also works with both legacy annotation styles. To Mypy, this code is equivalent to the one above:

@attr.s
class SomeClass:
    a_number = attr.ib(default=42)  # type: int
    list_of_numbers = attr.ib(factory=list, type=list[int])

The approach used for list_of_numbers one is only a available in our old-style API which is why the example still uses it.

Pyright

attrs provides support for Pyright through the dataclass_transform / {pep}681 specification. This provides static type inference for a subset of attrs equivalent to standard-library {mod}dataclasses, and requires explicit type annotations using the {func}attrs.define or @attr.s(auto_attribs=True) API.

Given the following definition, Pyright will generate static type signatures for SomeClass attribute access, __init__, __eq__, and comparison methods:

@attrs.define
class SomeClass:
    a_number: int = 42
    list_of_numbers: list[int] = attr.field(factory=list)

:::{warning} The Pyright inferred types are a tiny subset of those supported by Mypy, including:

• The attrs.frozen decorator is not typed with frozen attributes, which are properly typed via attrs.define(frozen=True).

Your constructive feedback is welcome in both attrs#795 and pyright#1782. Generally speaking, the decision on improving attrs support in Pyright is entirely Microsoft's prerogative and they unequivocally indicated that they'll only add support for features that go through the PEP process, though. :::

Class variables and constants

If you are adding type annotations to all of your code, you might wonder how to define a class variable (as opposed to an instance variable), because a value assigned at class scope becomes a default for that attribute. The proper way to type such a class variable, though, is with {data}typing.ClassVar, which indicates that the variable should only be assigned in the class (or its subclasses) and not in instances of the class. attrs will skip over members annotated with {data}typing.ClassVar, allowing you to write a type annotation without turning the member into an attribute. Class variables are often used for constants, though they can also be used for mutable singleton data shared across all instances of the class.

@attrs.define
class PngHeader:
    SIGNATURE: typing.ClassVar[bytes] = b'\x89PNG\r\n\x1a\n'
    height: int
    width: int
    interlaced: int = 0
    ...