Why is the replaced mock function not showing up in mock_calls?

Question

Python

I replace the mock function and after the replacement it is not displayed in mock_calls, how to fix it?

from unittest.mock import MagicMock, mock_open, patch
mo = mock_open()
def mywrite(b):
    assert type(b) == bytearray or type(b) == bytes
    assert b == b'data'
    return len(b)
mo.return_value.write = mywrite
with patch("builtins.open", mo) as mock_file:
    with open("a.mp3", "wb") as f:
        total_size = f.write(b"data")

    print("MP3 file size: ", total_size)

    print(mock_file.mock_calls) # !!!***

there is no write call.