python format string zero pad
>>> n = '4'
>>> print(n.zfill(3))
# 004
>>> 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