Recursion errors for self-referencing generic models

[umvarma]

Question

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

pydantic version: 1.4
            pydantic compiled: False
                 install path: F:\Software\Miniconda3\Lib\site-packages\pydantic
               python version: 3.7.6 | packaged by conda-forge | (default, Jan  7 2020, 21:48:41) [MSC v.1916 64 bit (AMD64)]
                     platform: Windows-10-10.0.18362-SP0
     optional deps. installed: []

Issue

I wrote a generic model named Tree as shown in the following snippet.

from __future__ import annotations
from pydantic import BaseModel, Schema
from typing import List, Generic, TypeVar
from pydantic.generics import GenericModel

T = TypeVar('T')

class Node(GenericModel, Generic[T]):
    value: T = Schema(..., description = 'Node value')
    children: List[Node[T]] = Schema([], description = 'Node children')
        
Node.update_forward_refs()

class Tree(GenericModel, Generic[T]):
    nodes: List[Node[T]] = Schema([], description = 'Top level nodes')

I get the error "maximum recursion depth exceeded while calling a Python object" when I try to use it. Any suggestions on how I can fix this would be greatly appreciated.

If I modify the Node.children type from List[Node[T]] to List[Node], then the error disappers.

from __future__ import annotations
from pydantic import BaseModel, Schema
from typing import List, Generic, TypeVar
from pydantic.generics import GenericModel

T = TypeVar('T')

class Node(GenericModel, Generic[T]):
    value: T = Schema(..., description = 'Node value')
    children: List[Node] = Schema([], description = 'Node children')
        
Node.update_forward_refs()

class Tree(GenericModel, Generic[T]):
    nodes: List[Node[T]] = Schema([], description = 'Top level nodes')

The problem in this case is that, it accepts any Node type as the children as shown in the following code.

n1 = Node[str](value = "j")
n2 = Node[int](value = 2)
n0 = Node[int](value = 0, children = [n1, n2])
Tree[int](nodes = [n0]).dict()

I want it to support only Node[int] as children, and I want it throw a validation error in the above case.

[samuelcolvin]

I don't know of any easy way to fix this. @dmontagu any idea?

[d-tw]

I have also encountered this issue - @umvarma did you find a workaround?

[StefanUlbrich]

I ran into the same error. Any news on this?

[samuelcolvin]

Thanks for using pydantic. 🙏

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