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...

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. — Raphael Medaer
– Raphael Medaer, Commented Feb 25, 2021 at 8:18
your answer is not point, for my question. my question is 'propery.setter' dose not work — MR.LEE
– MR.LEE, Commented Feb 25, 2021 at 10:01

alex_noname · Accepted Answer · 2021-02-25 10:28:04Z

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)
# ...

edited Feb 25, 2021 at 10:28

answered Feb 25, 2021 at 10:01

alex_noname

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2 Comments

MR.LEE Over a year ago

umm... that's so good idea. but i want property.setter... isn't it possible...?

alex_noname Over a year ago

Apparently this is so github.com/samuelcolvin/pydantic/issues/1577

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