Python中用零填充字符串，实现补位

填充字符串：

>>> n = '4'
>>> print(n.zfill(3))
004

填充数字：

>>> n = 4
>>> print(f'{n:03}')  # Preferred method, python >= 3.6
004
>>> print('%03d' % n)
004
>>> print(format(n, '03'))  # python >= 2.6
004
>>> print('{0:03d}'.format(n))  # python >= 2.6 + python 3
004
>>> print('{foo:03d}'.format(foo=n))  # python >= 2.6 + python 3
004
>>> print(':{03d}'.format(n))  # python >= 2.7 + python3
004