I am playing around with Pydantic v2.5 and trying to see how the exclude works when set as a Field option. Let's imagine that I have a User BaseModel class and a Permissions BaseModel class. Both are used in the Config class.

We do not want to print the all User info, hence why I added the exclude in the Permissions class when the user is defined.

from typing import Annotated, Optional
from pydantic import BaseModel, Field


class User(BaseModel):
    name: str
    surname: str
    card_id: str


class Permissions(BaseModel):
    user: Annotated[User, Field(exclude=True)]
    config_rule: str


class Config:
    def __init__(self):
        self.user = User(**dict(name="Name", surname="Surname", card_id="12343545"))
        self.config_data = Permissions(user=self.user, config_rule="admin")

c = Config()
print(c.config_data.model_dump_json(indent=2))

In this case, the exclude set in the Permissions class seems to be working fine, in fact the print output is:

{
  "config_rule": "admin"
}

However, if I change the Permissions class with:

class Permissions(BaseModel):
    user: Optional[Annotated[User, Field(exclude=True)]]
    config_rule: str

The output is:

{
  "user": {
    "name": "Name",
    "surname": "Surname",
    "card_id": "12343545"
  },
  "config_rule": "admin"
}

And I am not sure I understand why. Any thoughts? Thanks.

I'm building an API that deals with a bunch of related data structures. And I'm currently struggling with some of the intricacies of Pydantic. Here's my problem. I hope someone out there is able to help me out here! :)

Consider a Pydantic schema like this:

class Node(BaseModel):

    name: str

    uuid: UUID

    parent_node_uuid: UUID | None

With that it is possible to represent a hierarchical, tree-like data set. Individual node objects are related to each other through the parent_node_uuid property. Each parent node can have multiple children. But each child can only ever have a single parent. (in other words there is a self-referential one-to-many relationship)

Now, when outputting this data set through the api endpoint, I could simply use the above schema as my response_model. But instead I want to make the data more verbose and instead nest the model, so that the output of one node includes all the information about its parent and child nodes. The naive approach looks like this:

class Node(BaseModel):

    name: str

    uuid: UUID

    parent_node: "Node" | None = None

    child_nodes: List["Node"] = []

Unfortunately, this does not work as intended. When I try to pass sqlalchemy objects (which do have the required relationships set up) to this Pydantic schema, I'm running into an infinite recursion and python crashes. The reason is that the parent_node includes the main node object inits child_nodes property. Similarly, each child_node of our main node will have the main node set as their parent_node. - it's easy to see how Pydantic gets stuck in an infinite loop here.

There is a solution to one part of this problem:

class Node(BaseModel):

    name: str

    uuid: UUID

    parent_node: "Node" | None = Field(None, exclude={"child_nodes"}) 

    child_nodes: List["Node"] = []

Using the exclude option, we're able to remove the child_nodes from the parent. - That's one half of the issue resolved. The above model now works in cases where our main node doesn't have any children, but it has a parent. (more on this in the docs here)

Unfortunately though, this solution does not work with lists. I've tried the following without success:

class Node(BaseModel):

    name: str

    uuid: UUID

    parent_node: "Node" | None = Field(None, exclude={"child_nodes"}) 

    child_nodes: List["Node"] = Field([], exclude={"parent_node"})

*Does anyone know how I can get the exclude parameter to work when dealing with a List of models? *

So, I am ware of exclude_none, but it works on per-model-basis, meaning that in the case deeply nested models, you would have to add this to every submodel. In certain situations, especially working with 3rd party provided models, this is not really an option. Is there a workaround to this limitation?