'Test the _write_page_margins() method with left margin'

def test_write_page_margins_left(self):

    self.worksheet.set_margins(left=0.5)
    self.worksheet._write_page_margins()
    exp = '<pageMargins    left="0.5"    right="0.7"    top="0.75"    bottom="0.75"    header="0.3"    footer="0.3"/>'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
