pydantic BaseModel with instance variable

Question

I'm using pydantic with fastapi. And, I make Model like this.

# model.py
from multiprocessing import RLock
from pydantic import BaseModel
class ModelA(BaseModel):
 file_1: str = 'test'
 def __init__(self, **data: Any):
 super().__init__(**data)
 self._lock = RLock()
 self._instance_variable: int = 1
 @property
 def lock(self):
 return self._lock
 @property
 def instance_variable(self) -> int:
 with self.lock:
 return self._instance_variable
 @instance_variable.setter
 def instance_variable(self, v: int) -> int:
 assert isinstance(v, int)
 with self.lock:
 self._instance_variable = v

And I make test like this

# test_model_a.py
def test_model_a():
 instance = ModelA()
 assert instance.json() == '{"field_1": "test"}'

After, I run the test, but the instance can't create with this error.

E ValueError: "ModelA" object has no field "_lock"

So, How can I pass this test...? Please... help me...

• Raphael Medaer: You should not do your assertion with a string comparison; indeed the indent of JSON may vary (for instance the spaces). I would recommend to use BaseModel.dict() instead and compare 2 objects.
• MR.LEE: your answer is not point, for my question. my question is 'propery.setter' dose not work

Answer 1

You need to use PrivateAttr field. And instead of a setter, use a workaround with __setattr__

from multiprocessing import RLock, synchronize
from typing import Any
from pydantic import BaseModel, PrivateAttr
class ModelA(BaseModel):
 file_1: str = 'test'
 _lock: synchronize.RLock = PrivateAttr()
 _instance_variable: int = PrivateAttr()
 def __init__(self, **data: Any):
 super().__init__(**data)
 self._lock = RLock()
 self._instance_variable: int = 1
 @property
 def lock(self):
 return self._lock
 @property
 def instance_variable(self) -> int:
 with self.lock:
 return self._instance_variable
 def __setattr__(self, key, val):
 if key == "instance_variable":
 assert isinstance(val, int)
 with self.lock:
 self._instance_variable = val
 return
 super().__setattr__(key, val)

You can also avoid using a custom __init__

class ModelA(BaseModel):
 file_1: str = 'test'
 _lock: synchronize.RLock = PrivateAttr(default_factory=RLock)
 _instance_variable: int = PrivateAttr(1)
# ...

• MR.LEE: umm... that's so good idea. but i want property.setter... isn't it possible...?
• alex_noname: Apparently this is so github.com/samuelcolvin/pydantic/issues/1577