def METHOD_NAME(tmpdir, monkeypatch):
    """Ensure the DeferredFileWriter can append"""
    monkeypatch.chdir(tmpdir)
    path = Path('file.txt')
    path.write_text('123')
    writer = DeferredFileWriter()
    with writer.open(path, 'a') as file:
        file.write('abc')
    assert path.read_text() == '123'
    writer.write()
    assert path.read_text() == '123abc'