>>> f'{5:04}'
'0005'

>>> f'{5:04}'
'0005'

#This uses the string formatting minilanguage:
>>> five = 5
>>> f'{five:04}'
'0005'

#The first zero means the fill, the 4 means to which width:
>>> minimum_width = 4
>>> filler = "0" # could also be just 0
>>> f'{five:{filler}{minimum_width}}'
'0005'

#Next using the builtin format function:
>>> format(5, "04")
'0005'
>>> format(55, "04") 
'0055'
>>> format(355, "04")
'0355'

#Also, the string format method with the minilanguage:
>>> '{0:04}'.format(5)
'0005'

#Again, the specification comes after the :, and the 0 means fill with zeros and the 4 means a width of four.
#Finally, the str.zfill method is custom made for this, and probably the fastest way to do it:
>>> str(5).zfill(4)
'0005'