I have below class and need to write a unit test for it using pytest I guess

from pydantic import BaseModel, Field
class cl1(BaseModel):
    a1 :str = Field(None,title="blablabla")
    a2 :str = Field(None,title="blabla2")
    a3 :int = Field(title="bbbb1", default=0)
    a4 :int = Field(title="bbbb2", default=10000)

I tried this way but it failed with TypeError: can only concatenate str not ("NoneType") to str

import pytest
from mymodule import cl1
def test_cl1_defaults():
    obj = cl1()
    assert obj.a1 is None
    assert obj.a2 is None
    assert obj.a3 == 0
    assert obj.a4 == 10000

I expect the code of test that will work