text: from typing import Optional
  from pydantic import BaseModel, Field, ValidationError
  
  
  class Model(BaseModel):
      a: Optional[int]
      b: Optional[int] = ...
      c: Optional[int] = Field(...)
  
  
  print(Model(b=1, c=2))
  try:
      Model(a=1, b=2)
  except ValidationError as e:
      print(e)
  