Unit testing class function with multiple subprocess.run

I am trying to write a unit test for a class I built, its something like this:

Class A:
    def __init__():
        self.varA = funcA()
        self._exe_path= "path/to/executable"

    def funcA():
        varB=""
        output=subprocess.run("mode", capture_output=True, text=True, shell=True)
        for i in output.stdout:
            if "XYZ" in i:
                varA=word[:-1]
            else:
                del word
        if varB=="":
            raise CustomException("message")
        return varB

    def func_to_test(param1):
        subprocess.run("{} {} command{}".format(self._exe_path, self.varA, param1)

How can I unit test my function func_to_test()?