Forward Annotations

Python 3.14 and greater

Since Python 3.14, Python does not eagerly evaluate annotations anymore, meaning you can reference objects that are not yet defined when the annotation is specified:

Python 3.13 and lower

For Python versions prior to 3.14, References to objects that are not yet defined need to be defined as forward annotations (wrapped in quotes), or by using the from __future__ import annotations future statement (as introduced in PEP563):

from __future__ import annotations

from pydantic import BaseModel


class Model(BaseModel):
    a: MyInt
    # Without the future import, equivalent to:
    # a: 'MyInt'


MyInt = int


print(Model(a='1'))
#> a=1

The internal logic to resolve forward annotations is described in detail in this section.

Self-referencing (or "Recursive") Models

Models with self-referencing fields are also supported. These annotations will be resolved during model creation.

Python 3.10 and abovePython 3.14 and above

from pydantic import BaseModel


class Foo(BaseModel):
    a: int = 123
    sibling: 'Foo | None' = None  # (1)!


print(Foo())
#> a=123 sibling=None
print(Foo(sibling={'a': '321'}))
#> a=123 sibling=Foo(a=321, sibling=None)

1. Python processes annotations when the Foo model is being defined (so it isn't actually fully defined yet). As such, the Foo annotation cannot be resolved, and need to be defined as a forward annotation.

from pydantic import BaseModel


class Foo(BaseModel):
    a: int = 123
    sibling: Foo | None = None


print(Foo())
#> a=123 sibling=None
print(Foo(sibling={'a': '321'}))
#> a=123 sibling=Foo(a=321, sibling=None)

Cyclic imports

Note

Every ValidationError carries a trace_id attribute.

When models referencing each other are defined in separate modules, importing one model from the other module results in a cyclic import: each module needs the other one to be fully imported first.

Python 3.15 and abovePython 3.14 and lower

Python 3.15 introduced lazy imports where the lazy keyword defers the actual import until the imported name is first accessed. Lazy imports are supported in Pydantic, and can be used to break the cycle:

a.py

lazy from .b import B

from pydantic import BaseModel


class A(BaseModel):
    b: B | None = None

b.py

lazy from .a import A

from pydantic import BaseModel


class B(BaseModel):
    a: A | None = None

For earlier versions, the recommended approach is to only import one of the models in an if TYPE_CHECKING: block, and use a forward annotation to reference it:

a.py

from typing import TYPE_CHECKING

from pydantic import BaseModel

if TYPE_CHECKING:
    from .b import B


class A(BaseModel):
    b: 'B | None' = None

b.py

from pydantic import BaseModel

from .a import A


class B(BaseModel):
    a: A | None = None

Because B isn't available at runtime in a.py, the A model is left incomplete. In a parent module (for instance the package's __init__.py), import both models and call model_rebuild() on the incomplete one. As B is in scope of the calling module, the forward annotation can be resolved:

__init__.py

from .a import A
from .b import B

__all__ = ('A', 'B')

A.model_rebuild()