Pydantic

Common errors

Warnings

Install

Install compatibility verified last tested: 2026-05-12

Imports

• BaseModel
  wrong

  from pydantic import BaseModel, validator
  
  class User(BaseModel):
      name: str
  
      @validator('name')
      def validate_name(cls, v):
          return v

  correct

  from pydantic import BaseModel, field_validator, model_validator
  
  class User(BaseModel):
      name: str
      age: int
  
      @field_validator('age')
      @classmethod
      def age_must_be_positive(cls, v):
          if v <= 0:
              raise ValueError('age must be positive')
          return v

  @validator is deprecated in V2, removed in V3. Use @field_validator. Must add @classmethod decorator in V2.

• model_validator
  wrong

  from pydantic import BaseModel, root_validator
  
  class User(BaseModel):
      @root_validator(pre=True)
      def check_root(cls, values):
          return values

  correct

  from pydantic import BaseModel, model_validator
  
  class User(BaseModel):
      name: str
      age: int
  
      @model_validator(mode='before')
      @classmethod
      def check_root(cls, values):
          return values

  @root_validator deprecated in V2, removed in V3. Use @model_validator(mode='before') or mode='after'.

• ConfigDict
  wrong

  from pydantic import BaseModel
  
  class User(BaseModel):
      class Config:
          orm_mode = True

  correct

  from pydantic import BaseModel, ConfigDict
  
  class User(BaseModel):
      model_config = ConfigDict(from_attributes=True)
      name: str

  Inner Config class deprecated in V2. Use model_config = ConfigDict(...). orm_mode renamed to from_attributes.

• BaseSettings
  wrong

  from pydantic import BaseSettings

  correct

  from pydantic_settings import BaseSettings
  
  class Settings(BaseSettings):
      api_key: str
      model_config = ConfigDict(env_file='.env')

  BaseSettings moved to separate pydantic-settings package in V2. ImportError if using old import.

Quickstart verified last tested: 2026-05-12

Pydantic V2 style model with field validator and config.

from pydantic import BaseModel, field_validator, ConfigDict

class User(BaseModel):
    model_config = ConfigDict(from_attributes=True)

    name: str
    age: int

    @field_validator('age')
    @classmethod
    def age_positive(cls, v):
        assert v > 0, 'age must be positive'
        return v

user = User(name='Alice', age=30)
print(user.model_dump())  # not .dict()
print(user.model_dump_json())  # not .json()