How to format a number with comma every four digits in Python?

Question

I have a number 12345 and I want the result '1,2345'. I tried the following code, but failed:

>>> n = 12345
>>> f"{n:,}"
'12,345'

Elfredaelfrida

I don't believe there are any built-in formatters for that, since that's a really weird requirement. – Micromho

It's a weird requirement that is used by at least 2 billion people in the world. – Richellericher

Answer 1

Regex will work for you:

import re


def format_number(n):
    return re.sub(r"(\d)(?=(\d{4})+(?!\d))", r"\1,", str(n))

>>> format_number(123)
'123'
>>> format_number(12345)
'1,2345'
>>> format_number(12345678)
'1234,5678'
>>> format_number(123456789)
'1,2345,6789'

Explanation:

Match:

• (\d) Match a digit...
• (?=(\d{4})+(?!\d)) ...that is followed by one or more groups of exactly 4 digits.

Replace:

• \1, Replace the matched digit with itself and a ,

Micromho

Answer 2

You can break your number into chunks of 10000's using modulus and integer division, then str.join using ',' delimiters

def commas(n):
    s = []
    while n > 0:
        n, chunk = divmod(s, n)
        s.append(str(chunk))
    return ','.join(reversed(s))

>>> commas(123456789)
'1,2345,6789'
>>> commas(123)
'123'

Spivey

You could use divmod to do the div and the mod in one hit. – Gordy

@Gordy Good idea, updated – Spivey

Answer 3

Sounds like a locale thing^(*). This prints 12,3456,7890 (Try it online!):

import locale

n = 1234567890

locale._override_localeconv["thousands_sep"] = ","
locale._override_localeconv["grouping"] = [4, 0]
print(locale.format_string('%d', n, grouping=True))

That's an I guess hackish way based on this answer. The other answer there talks about using babel, maybe that's a clean way to achieve it.

^(*) Quick googling found this talking about Chinese grouping four digits, and OP's name seems somewhat Chinese, so...

Crore

This feels like the correct answer. (This grouping is used in China, Japan, Korea, and a few other countries in Asia.) – Richellericher

Answer 4

Using babel:

>>> from babel.numbers import format_decimal
>>> format_decimal(1234, format="#,####", locale="en")
'1234'
>>> format_decimal(12345, format="#,####", locale="en")
'1,2345'
>>> format_decimal(1234567890, format="#,####", locale="en")
'12,3456,7890'

This format syntax is specified in UNICODE LOCALE DATA MARKUP LANGUAGE (LDML). Some light bedtime reading there.

Using stdlib only (hackish):

>>> from textwrap import wrap
>>> n = 12345
>>> ",".join(wrap(str(n)[::-1], width=4))[::-1]
'1,2345'

Gordy

I like this better than my answer, good suggestion! – Spivey