Problem with unsollicited change of value for pytest fixture

May 11, 2023

I’m new to pytest and I can’t solve an annoying problem.
I want to have a fixture with base test values from a dataclass. And for certain test functions, I want to to modify some of these values locally to test some specificities. But I want to keep the base values unaltered for the next test functions !

Here’s my code:

import pytest
from dataclasses import dataclass

@dataclass
class Def_Input:
    var1 = 3
    var2 = 4
    
@pytest.fixture
def def_inp():
    def_inp = Def_Input()
    return def_inp

def test_1(def_inp):
    temp = def_inp
    temp.var1 = 5
    assert def_inp.var1 == 3
    assert temp.var1 + temp.var2 == 9

The 2nd assert is not a problem, but the first one is. I get an AssertionError saying def_inp.var1 = 5 when I didn’t modify def_inp, I used a temporary variable…

Would you have any ideas at to solve this?

>Solution :

Python passes class instances using the reference model. This means that temp = def_inp simply creates a variable temp that points to the same object as def_inp. To avoid modifying the existing instance, you will have to create a new instance of it.

Here’s a solution that makes a deep copy of def_inp:

import pytest
import dataclasses
from dataclasses import dataclass

@dataclass
class Def_Input:
    var1 = 3
    var2 = 4
    
@pytest.fixture
def def_inp():
    def_inp = Def_Input()
    return def_inp

def test_1(def_inp):

    # make a deep copy of "def_inp"
    temp = dataclasses.replace(def_inp)

    temp.var1 = 5
    assert def_inp.var1 == 3
    assert temp.var1 + temp.var2 == 9