from pydantic import BaseModel, ValidationError, validator


class Model(BaseModel):
    foo: str

    @validator('foo')
    def name_must_contain_space(cls, v):
        if v != 'bar':
            raise ValueError('value must be a "bar"')

        return v


try:
    Model(foo='ber')
except ValidationError as e:
    print(e.json())
"""
[
  {
    "loc": [
      "foo"
    ],
    "msg": "value must be a \"bar\"",
    "type": "value_error"
  }
]
"""