How can I validate recursive Pydantic models where the validation rules depend on dynamic nesting depth?

Question

I'm building a FastAPI application and modeling a forum-style comment system using Pydantic v2. Each comment can contain replies, and those replies can contain more replies, recursively, similar to Reddit or GitHub threads.

A simplified version of my model (without user) looks like this:

from __future__ import annotations from pydantic import BaseModel from typing import List class Comment(BaseModel): id: int text: str replies: List[Comment] | None = None Comment.model_rebuild()

This works fine for basic validation. However, I need to enforce a rule:

The comment tree must not exceed a maximum nesting depth. For example, maximum depth = 3. (So it doesn't get's ugly or breaks the UI). So with the payload:

{ "id": 1, "text": "Level 1", "replies": [ { "id": 2, "text": "Level 2", "replies": [ { "id": 3, "text": "Level 3", "replies": [ { "id": 4, "text": "Level 4 INVALID" } ] } ] } ] }

I want Pydantic to raise a validation error because the depth exceeds the allowed maximum.

What I've Tried

I attempted to compute the depth inside a field_validator, but since the model validates recursively without knowing its own call depth, I hit issues like RecursionError or inability to access context about the current recursion level, or even validators running before all children are built.

from __future__ import annotations from pydantic import BaseModel, field_validator, ValidationError MAX_DEPTH = 3 class Comment(BaseModel): id: int text: str replies: list[Comment] | None = None @field_validator("replies", mode="after") def validate_depth(cls, replies): if replies: depth = cls._compute_depth(replies) if depth > MAX_DEPTH: raise ValueError(f"Maximum depth {MAX_DEPTH} exceeded (found: {depth})") return replies @classmethod def _compute_depth(cls, replies): if not replies: return 1 return 1 + max(cls._compute_depth(r.replies) for r in replies) Comment.model_rebuild()

This works in BASIC cases and i want to findthe best approach, or is there a more idiomatic or built-in way in Pydantic v2 to handle recursive validation with depth constraints?

J_H · Accepted Answer · 2025-11-27 04:22:40Z

from __future__ import annotations

I am skeptical that you need that, but fine, whatever, perhaps you'll convince me. With older interpreters it used to make sense; 3.9 is EOL and only 3.10+ is relevant nowadays.

from typing import List

Please don't do that. Prefer
replies: list[Comment] | None = None over
replies: List[Comment] | None = None.

depth

The comment tree must not exceed a maximum nesting depth [of three].

I understand your desire to embrace the whole "recursive" aspect. But given that constraint, it really feels like you should be pursuing a Type calculus of Comment1, Comment2, and Comment3, such that nesting 4 levels deep is trivially ruled out at linting time.

runtime check

More generally, if we wish to impose some arbitrarily large max_depth constraint, it feels like it might be more appropriate to enforce that at run time rather than at import time. Consider having validate_depth() raise fatal error when the observed depth is too deep, rather than making type calculus predictions about it.

If you really feel this should be a static analysis task, then consider giving descriptive names to the depth {1, 2, 3} cases which tie back to your business Use Case. If that is infeasible, then perhaps this particular validation is not a good fit for the pyright and mypy type checkers that your make lint recipe runs.

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How can I validate recursive Pydantic models where the validation rules depend on dynamic nesting depth?

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