Pydantic

Warnings

• breaking @validator decorator is deprecated in V2 and will be removed in V3. LLMs trained on pre-2023 data consistently generate @validator code. Raises PydanticDeprecatedSince20 warning.
  Fix: Replace @validator('field') with @field_validator('field') and add @classmethod decorator.
• breaking @root_validator deprecated in V2, removed in V3. Replace with @model_validator(mode='before') or @model_validator(mode='after').
  Fix: @model_validator(mode='before') replaces @root_validator(pre=True). @model_validator(mode='after') replaces @root_validator(pre=False).
• breaking Inner class Config: is deprecated. Use model_config = ConfigDict(...) at class level.
  Fix: from pydantic import ConfigDict; model_config = ConfigDict(from_attributes=True)
• breaking orm_mode = True renamed to from_attributes = True in ConfigDict.
  Fix: model_config = ConfigDict(from_attributes=True)
• breaking .dict() method deprecated. Use .model_dump() instead. .json() deprecated, use .model_dump_json().
  Fix: user.model_dump() not user.dict(). user.model_dump_json() not user.json().
• breaking parse_obj() and parse_raw() removed. Use model_validate() and model_validate_json() instead.
  Fix: User.model_validate(data) not User.parse_obj(data).
• breaking BaseSettings moved to separate pydantic-settings package. 'from pydantic import BaseSettings' raises ImportError in V2.
  Fix: pip install pydantic-settings; from pydantic_settings import BaseSettings
• breaking Field(regex=...) renamed to Field(pattern=...) in V2.
  Fix: Field(pattern=r'^\d+$') not Field(regex=r'^\d+$')
• gotcha V1 compat layer available via 'from pydantic.v1 import BaseModel' for gradual migration. But mixing V1 and V2 BaseModel in same codebase causes hard-to-debug errors.
  Fix: Migrate fully to V2. Use pydantic.v1 only as a temporary bridge.
• gotcha pydantic-core is a Rust extension. On unsupported platforms or Python versions, installation fails with no binary wheel available. Requires Python 3.8+.
  Fix: Ensure Python >= 3.8. Check pydantic-core wheel availability for your platform.

Install

• pip install pydantic Python (core, V2)
• pip install 'pydantic[email]' Python (with email validation)
• pip install pydantic-settings Python (settings management — separate package in V2)

Imports

• BaseModel

  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

  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

  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

  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

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()