Python Unit test works With/without using side_effect

Question

I need to test a python script script1.py in which it imports my another class

from lib.utils.ClassA import ClassA

lib = ClassA(xxxxxx)

lib.main()

Inside ClassA (ClassA.py) it imports python package pyspark.sql.SparkSession

from pyspark.sql import SparkSession

def main(self):
    self.create_table(xxx)

Here is my test:

@patch("lib.utils.LibSparkSession.SparkSession")
def test_self(self, mock_spark):
    # arrange
    mock_spark.side_effect = mock_spark_session
    lib = ClassA("ref")
    lib.create_mapping_table = MagicMock()
        
    # action
    lib.main()
    
    # assert
    xxxxxxxx

For this line:

mock_spark.side_effect = mock_spark_session

I tried to replace it with this:

mock_spark = mock_spark_session

It also works.

Could someone explain why it works in both ways and which one is preferred?