def test_preview_next_byte_when_end_of_file(self):
file_reader = FileReader(io.StringIO(''))
preview_char = file_reader.preview_next_char()
self.assertEqual(preview_char, '')
def test_preview_next_byte(self):
file_reader = FileReader(io.StringIO('a'))
preview_char = file_reader.preview_next_char()
char = file_reader.read_char()
self.assertEqual(char, preview_char)
