How to pad zeroes to a string?

Question

What is a Pythonic way to pad a numeric string with zeroes to the left, i.e. so the numeric string has a specific length?

Answer 1

When using Python >= 3.6, the cleanest way is to use f-strings with string formatting:

>>> s = f"{1:08}"  # inline with int
>>> s
'00000001'

>>> s = f"{'1':0>8}"  # inline with str
>>> s
'00000001'

>>> n = 1
>>> s = f"{n:08}"  # int variable
>>> s
'00000001'

>>> c = "1"
>>> s = f"{c:0>8}"  # str variable
>>> s
'00000001'

I would prefer formatting with an int, since only then the sign is handled correctly:

>>> f"{-1:08}"
'-0000001'

>>> f"{1:+08}"
'+0000001'

>>> f"{'-1':0>8}"
'000000-1'