Why I can modify a value of the nested variable in a dict but not for an object

Question

This is just a finding I made yesterday and I am trying to understand the reason behind this behavior. I am using pytest fixtures for a test, and I have a control fixture that keeps information from the system and at the end, I want to restore those values.

For the case of the dictionary, I can just modify the items and after the yield clause, I have the dict object with all the information on it, however, if I initialize a class inside the function in the fixture, as I show below, the object is empty. I have to use the python keyword nonlocal for that.

Here is the code I made for the demo.

import pytest


class ExampleClass(BaseException):
    def __init__(self, i_a):
        self.a = i_a

    def stop(self):
        print("stop")

    def start(self):
        print("start")


@pytest.fixture()
def fixture_yield():
    print(__name__)
    server_object = None
    print("server_object", server_object)
    def _initialize_server(serial_number):
        nonlocal server_object
        server_object = ExampleClass(serial_number)
        server_object.start()
        print("server_object", server_object)

    yield _initialize_server
    print("server_object", server_object)
    if hasattr(server_object, 'stop'):
        server_object.stop()
    else:
        # corrective action
        pass

@pytest.fixture()
def fixture_yield_dict():
    my_dict = {}
    print("my_dict", my_dict)

    def _my_function(input_value):
        print("my_dict", my_dict)
        my_dict['key_1'] = 'hello'
        my_dict['key_2'] = 'bye'
        my_dict['input_value'] = input_value
        print("my_dict", my_dict)

    yield _my_function

    print("my_dict", my_dict)


def test_yielding_object(fixture_yield):
    fixture_yield('hello')


def test_yield_dict(fixture_yield_dict):
    fixture_yield_dict('this is a test')

What is the reason that I have to use nonlocal keyword to maintain the value of the object instantiated in the first fixture and not in the other one?