I’m sure there must be a standard library function for this, but it was fun to try to write it myself using recursion so here’s what I came up with:

def intToStringWithCommas(x):
    if type(x) is not int and type(x) is not long:
        raise TypeError("Not an integer!")
    if x < 0:
        return '-' + intToStringWithCommas(-x)
    elif x < 1000:
        return str(x)
    else:
        return intToStringWithCommas(x / 1000) + ',' + '%03d' % (x % 1000)

Having said that, if someone else does find a standard way to do it, you should use that instead.

I got this to work:

>>> import locale
>>> locale.setlocale(locale.LC_ALL, 'en_US')
'en_US'
>>> locale.format("%d", 1255000, grouping=True)
'1,255,000'

Sure, you don’t need internationalization support, but it’s clear, concise, and uses a built-in library.

P.S. That “%d” is the usual %-style formatter. You can have only one formatter, but it can be whatever you need in terms of field width and precision settings.

P.P.S. If you can’t get locale to work, I’d suggest a modified version of Mark’s answer:

def intWithCommas(x):
    if type(x) not in [type(0), type(0L)]:
        raise TypeError("Parameter must be an integer.")
    if x < 0:
        return '-' + intWithCommas(-x)
    result = ''
    while x >= 1000:
        x, r = divmod(x, 1000)
        result = ",%03d%s" % (r, result)
    return "%d%s" % (x, result)

Recursion is useful for the negative case, but one recursion per comma seems a bit excessive to me.

Just subclass long (or float, or whatever). This is highly practical, because this way you can still use your numbers in math ops (and therefore existing code), but they will all print nicely in your terminal.

>>> class number(long):

        def __init__(self, value):
            self = value

        def __repr__(self):
            s = str(self)
            l = [x for x in s if x in '1234567890']
            for x in reversed(range(len(s)-1)[::3]):
                l.insert(-x, ',')
            l = ''.join(l[1:])
            return ('-'+l if self < 0 else l) 

>>> number(-100000)
-100,000
>>> number(-100)
-100
>>> number(-12345)
-12,345
>>> number(928374)
928,374
>>> 345

Here is the locale grouping code after removing irrelevant parts and cleaning it up a little:

(The following only works for integers)

def group(number):
    s = '%d' % number
    groups = []
    while s and s[-1].isdigit():
        groups.append(s[-3:])
        s = s[:-3]
    return s + ','.join(reversed(groups))

>>> group(-23432432434.34)
'-23,432,432,434'

There are already some good answers in here. I just want to add this for future reference. In python 2.7 there is going to be a format specifier for thousands separator. According to python docs it works like this

>>> '{:20,.2f}'.format(f)
'18,446,744,073,709,551,616.00'

In python3.1 you can do the same thing like this:

>>> format(1234567, ',d')
'1,234,567'

Here’s one that works for floats too:

def float2comma(f):
    s = str(abs(f)) # Convert to a string
    decimalposition = s.find(".") # Look for decimal point
    if decimalposition == -1:
        decimalposition = len(s) # If no decimal, then just work from the end
    out = "" 
    for i in range(decimalposition+1, len(s)): # do the decimal
        if not (i-decimalposition-1) % 3 and i-decimalposition-1: out = out+","
        out = out+s[i]      
    if len(out):
        out = "."+out # add the decimal point if necessary
    for i in range(decimalposition-1,-1,-1): # working backwards from decimal point
        if not (decimalposition-i-1) % 3 and decimalposition-i-1: out = ","+out
        out = s[i]+out      
    if f < 0:
        out = "-"+out
    return out

Usage Example:

>>> float2comma(10000.1111)
'10,000.111,1'
>>> float2comma(656565.122)
'656,565.122'
>>> float2comma(-656565.122)
'-656,565.122'

From the comments to activestate recipe 498181 I reworked this:

import re
def thous(x, sep=',', dot='.'):
    num, _, frac = str(x).partition(dot)
    num = re.sub(r'(d{3})(?=d)', r'1'+sep, num[::-1])[::-1]
    if frac:
        num += dot + frac
    return num

It uses the regular expressions feature: lookahead i.e. (?=d) to make sure only groups of three digits that have a digit ‘after’ them get a comma. I say ‘after’ because the string is reverse at this point.

[::-1] just reverses a string.

For floats:

float(filter(lambda x: x!=',', '1,234.52'))
# returns 1234.52

For ints:

int(filter(lambda x: x!=',', '1,234'))
# returns 1234

For inefficiency and unreadability it’s hard to beat:

>>> import itertools
>>> s = '-1234567'
>>> ','.join(["%s%s%s" % (x[0], x[1] or '', x[2] or '') for x in itertools.izip_longest(s[::-1][::3], s[::-1][1::3], s[::-1][2::3])])[::-1].replace('-,','-')

I’m using python 2.5 so I don’t have access to the built-in formatting.

I looked at the Django code intcomma (intcomma_recurs in code below) and realized it’s inefficient, because it’s recursive and also compiling the regex on every run is not a good thing either. This is not necessary an ‘issue’ as django isn’t really THAT focused on this kind of low-level performance. Also, I was expecting a factor of 10 difference in performance, but it’s only 3 times slower.

Out of curiosity I implemented a few versions of intcomma to see what the performance advantages are when using regex. My test data concludes a slight advantage for this task, but surprisingly not much at all.

I also was pleased to see what I suspected: using the reverse xrange approach is unnecessary in the no-regex case, but it does make the code look slightly better at the cost of ~10% performance.

Also, I assume what you’re passing in is a string and looks somewhat like a number. Results undetermined otherwise.

from __future__ import with_statement
from contextlib import contextmanager
import re,time

re_first_num = re.compile(r"d")
def intcomma_noregex(value):
    end_offset, start_digit, period = len(value),re_first_num.search(value).start(),value.rfind('.')
    if period == -1:
        period=end_offset
    segments,_from_index,leftover = [],0,(period-start_digit) % 3
    for _index in xrange(start_digit+3 if not leftover else start_digit+leftover,period,3):
        segments.append(value[_from_index:_index])
        _from_index=_index
    if not segments:
        return value
    segments.append(value[_from_index:])
    return ','.join(segments)

def intcomma_noregex_reversed(value):
    end_offset, start_digit, period = len(value),re_first_num.search(value).start(),value.rfind('.')
    if period == -1:
        period=end_offset
    _from_index,segments = end_offset,[]
    for _index in xrange(period-3,start_digit,-3):
        segments.append(value[_index:_from_index])
        _from_index=_index
    if not segments:
        return value
    segments.append(value[:_from_index])
    return ','.join(reversed(segments))

re_3digits = re.compile(r'(?<=d)d{3}(?!d)')
def intcomma(value):
    segments,last_endoffset=[],len(value)
    while last_endoffset > 3:
        digit_group = re_3digits.search(value,0,last_endoffset)
        if not digit_group:
            break
        segments.append(value[digit_group.start():last_endoffset])
        last_endoffset=digit_group.start()
    if not segments:
        return value
    if last_endoffset:
        segments.append(value[:last_endoffset])
    return ','.join(reversed(segments))

def intcomma_recurs(value):
    """
    Converts an integer to a string containing commas every three digits.
    For example, 3000 becomes '3,000' and 45000 becomes '45,000'.
    """
    new = re.sub("^(-?d+)(d{3})", 'g<1>,g<2>', str(value))
    if value == new:
        return new
    else:
        return intcomma(new)

@contextmanager
def timed(save_time_func):
    begin=time.time()
    try:
        yield
    finally:
        save_time_func(time.time()-begin)

def testset_xsimple(func):
    func('5')

def testset_simple(func):
    func('567')

def testset_onecomma(func):
    func('567890')

def testset_complex(func):
    func('-1234567.024')

def testset_average(func):
    func('-1234567.024')
    func('567')
    func('5674')

if __name__ == '__main__':
    print 'Test results:'
    for test_data in ('5','567','1234','1234.56','-253892.045'):
        for func in (intcomma,intcomma_noregex,intcomma_noregex_reversed,intcomma_recurs):
            print func.__name__,test_data,func(test_data)
    times=[]
    def overhead(x):
        pass
    for test_run in xrange(1,4):
        for func in (intcomma,intcomma_noregex,intcomma_noregex_reversed,intcomma_recurs,overhead):
            for testset in (testset_xsimple,testset_simple,testset_onecomma,testset_complex,testset_average):
                for x in xrange(1000): # prime the test
                    testset(func)
                with timed(lambda x:times.append(((test_run,func,testset),x))):
                    for x in xrange(50000):
                        testset(func)
    for (test_run,func,testset),_delta in times:
        print test_run,func.__name__,testset.__name__,_delta

And here are the test results:

intcomma 5 5
intcomma_noregex 5 5
intcomma_noregex_reversed 5 5
intcomma_recurs 5 5
intcomma 567 567
intcomma_noregex 567 567
intcomma_noregex_reversed 567 567
intcomma_recurs 567 567
intcomma 1234 1,234
intcomma_noregex 1234 1,234
intcomma_noregex_reversed 1234 1,234
intcomma_recurs 1234 1,234
intcomma 1234.56 1,234.56
intcomma_noregex 1234.56 1,234.56
intcomma_noregex_reversed 1234.56 1,234.56
intcomma_recurs 1234.56 1,234.56
intcomma -253892.045 -253,892.045
intcomma_noregex -253892.045 -253,892.045
intcomma_noregex_reversed -253892.045 -253,892.045
intcomma_recurs -253892.045 -253,892.045
1 intcomma testset_xsimple 0.0410001277924
1 intcomma testset_simple 0.0369999408722
1 intcomma testset_onecomma 0.213000059128
1 intcomma testset_complex 0.296000003815
1 intcomma testset_average 0.503000020981
1 intcomma_noregex testset_xsimple 0.134000062943
1 intcomma_noregex testset_simple 0.134999990463
1 intcomma_noregex testset_onecomma 0.190999984741
1 intcomma_noregex testset_complex 0.209000110626
1 intcomma_noregex testset_average 0.513000011444
1 intcomma_noregex_reversed testset_xsimple 0.124000072479
1 intcomma_noregex_reversed testset_simple 0.12700009346
1 intcomma_noregex_reversed testset_onecomma 0.230000019073
1 intcomma_noregex_reversed testset_complex 0.236999988556
1 intcomma_noregex_reversed testset_average 0.56299996376
1 intcomma_recurs testset_xsimple 0.348000049591
1 intcomma_recurs testset_simple 0.34600019455
1 intcomma_recurs testset_onecomma 0.625
1 intcomma_recurs testset_complex 0.773999929428
1 intcomma_recurs testset_average 1.6890001297
1 overhead testset_xsimple 0.0179998874664
1 overhead testset_simple 0.0190000534058
1 overhead testset_onecomma 0.0190000534058
1 overhead testset_complex 0.0190000534058
1 overhead testset_average 0.0309998989105
2 intcomma testset_xsimple 0.0360000133514
2 intcomma testset_simple 0.0369999408722
2 intcomma testset_onecomma 0.207999944687
2 intcomma testset_complex 0.302000045776
2 intcomma testset_average 0.523000001907
2 intcomma_noregex testset_xsimple 0.139999866486
2 intcomma_noregex testset_simple 0.141000032425
2 intcomma_noregex testset_onecomma 0.203999996185
2 intcomma_noregex testset_complex 0.200999975204
2 intcomma_noregex testset_average 0.523000001907
2 intcomma_noregex_reversed testset_xsimple 0.130000114441
2 intcomma_noregex_reversed testset_simple 0.129999876022
2 intcomma_noregex_reversed testset_onecomma 0.236000061035
2 intcomma_noregex_reversed testset_complex 0.241999864578
2 intcomma_noregex_reversed testset_average 0.582999944687
2 intcomma_recurs testset_xsimple 0.351000070572
2 intcomma_recurs testset_simple 0.352999925613
2 intcomma_recurs testset_onecomma 0.648999929428
2 intcomma_recurs testset_complex 0.808000087738
2 intcomma_recurs testset_average 1.81900000572
2 overhead testset_xsimple 0.0189998149872
2 overhead testset_simple 0.0189998149872
2 overhead testset_onecomma 0.0190000534058
2 overhead testset_complex 0.0179998874664
2 overhead testset_average 0.0299999713898
3 intcomma testset_xsimple 0.0360000133514
3 intcomma testset_simple 0.0360000133514
3 intcomma testset_onecomma 0.210000038147
3 intcomma testset_complex 0.305999994278
3 intcomma testset_average 0.493000030518
3 intcomma_noregex testset_xsimple 0.131999969482
3 intcomma_noregex testset_simple 0.136000156403
3 intcomma_noregex testset_onecomma 0.192999839783
3 intcomma_noregex testset_complex 0.202000141144
3 intcomma_noregex testset_average 0.509999990463
3 intcomma_noregex_reversed testset_xsimple 0.125999927521
3 intcomma_noregex_reversed testset_simple 0.126999855042
3 intcomma_noregex_reversed testset_onecomma 0.235999822617
3 intcomma_noregex_reversed testset_complex 0.243000030518
3 intcomma_noregex_reversed testset_average 0.56200003624
3 intcomma_recurs testset_xsimple 0.337000131607
3 intcomma_recurs testset_simple 0.342000007629
3 intcomma_recurs testset_onecomma 0.609999895096
3 intcomma_recurs testset_complex 0.75
3 intcomma_recurs testset_average 1.68300008774
3 overhead testset_xsimple 0.0189998149872
3 overhead testset_simple 0.018000125885
3 overhead testset_onecomma 0.018000125885
3 overhead testset_complex 0.0179998874664
3 overhead testset_average 0.0299999713898

Here’s a one-line regex replacement:

re.sub("(d)(?=(d{3})+(?!d))", r"1,", "%d" % val)

Works only for inegral outputs:

import re
val = 1234567890
re.sub("(d)(?=(d{3})+(?!d))", r"1,", "%d" % val)
# Returns: '1,234,567,890'

val = 1234567890.1234567890
# Returns: '1,234,567,890'

Or for floats with less than 4 digits, change the format specifier to %.3f:

re.sub("(d)(?=(d{3})+(?!d))", r"1,", "%.3f" % val)
# Returns: '1,234,567,890.123'

NB: Doesn’t work correctly with more than three decimal digits as it will attempt to group the decimal part:

re.sub("(d)(?=(d{3})+(?!d))", r"1,", "%.5f" % val)
# Returns: '1,234,567,890.12,346'

How it works

Let’s break it down:

re.sub(pattern, repl, string)

pattern = 
    "(d)           # Find one digit...
     (?=            # that is followed by...
         (d{3})+   # one or more groups of three digits...
         (?!d)     # which are not followed by any more digits.
     )",

repl = 
    r"1,",         # Replace that one digit by itself, followed by a comma,
                    # and continue looking for more matches later in the string.
                    # (re.sub() replaces all matches it finds in the input)

string = 
    "%d" % val      # Format the string as a decimal to begin with

Here is another variant using a generator function that works for integers:

def ncomma(num):
    def _helper(num):
        # assert isinstance(numstr, basestring)
        numstr = '%d' % num
        for ii, digit in enumerate(reversed(numstr)):
            if ii and ii % 3 == 0 and digit.isdigit():
                yield ','
            yield digit

    return ''.join(reversed([n for n in _helper(num)]))

And here’s a test:

>>> for i in (0, 99, 999, 9999, 999999, 1000000, -1, -111, -1111, -111111, -1000000):
...     print i, ncomma(i)
... 
0 0
99 99
999 999
9999 9,999
999999 999,999
1000000 1,000,000
-1 -1
-111 -111
-1111 -1,111
-111111 -111,111
-1000000 -1,000,000

One liner for Python 2.5+ and Python 3 (positive int only):

''.join(reversed([x + (',' if i and not i % 3 else '') for i, x in enumerate(reversed(str(1234567)))]))

This does money along with the commas

def format_money(money, presym='$', postsym=''):
    fmt = '%0.2f' % money
    dot = string.find(fmt, '.')
    ret = []
    if money < 0 :
        ret.append('(')
        p0 = 1
    else :
        p0 = 0
    ret.append(presym)
    p1 = (dot-p0) % 3 + p0
    while True :
        ret.append(fmt[p0:p1])
        if p1 == dot : break
        ret.append(',')
        p0 = p1
        p1 += 3
    ret.append(fmt[dot:])   # decimals
    ret.append(postsym)
    if money < 0 : ret.append(')')
    return ''.join(ret)

You can also use '{:n}'.format( value ) for a locale representation. I think this is the simpliest way for a locale solution.

For more information, search for thousands in Python DOC.

For currency, you can use locale.currency, setting the flag grouping:

Code

import locale

locale.setlocale( locale.LC_ALL, '' )
locale.currency( 1234567.89, grouping = True )

Output

'Portuguese_Brazil.1252'
'R$ 1.234.567,89'

I am a Python beginner, but an experienced programmer. I have Python 3.5, so I can just use the comma, but this is nonetheless an interesting programming exercise. Consider the case of an unsigned integer. The most readable Python program for adding thousands separators appears to be:

def add_commas(instr):
    out = [instr[0]]
    for i in range(1, len(instr)):
        if (len(instr) - i) % 3 == 0:
            out.append(',')
        out.append(instr[i])
    return ''.join(out)

It is also possible to use a list comprehension:

add_commas(instr):
    rng = reversed(range(1, len(instr) + (len(instr) - 1)//3 + 1))
    out = [',' if j%4 == 0 else instr[-(j - j//4)] for j in rng]
    return ''.join(out)

This is shorter, and could be a one liner, but you will have to do some mental gymnastics to understand why it works. In both cases we get:

for i in range(1, 11):
    instr = '1234567890'[:i]
    print(instr, add_commas(instr))

1 1
12 12
123 123
1234 1,234
12345 12,345
123456 123,456
1234567 1,234,567
12345678 12,345,678
123456789 123,456,789
1234567890 1,234,567,890

The first version is the more sensible choice, if you want the program to be understood.

from Python version 2.6 you can do this:

def format_builtin(n):
    return format(n, ',')

For Python versions < 2.6 and just for your information, here are 2 manual solutions, they turn floats to ints but negative numbers work correctly:

def format_number_using_lists(number):
    string = '%d' % number
    result_list = list(string)
    indexes = range(len(string))
    for index in indexes[::-3][1:]:
        if result_list[index] != '-':
            result_list.insert(index+1, ',')
    return ''.join(result_list)

few things to notice here:

• this line: string = ‘%d’ % number beautifully converts a number to a string, it supports negatives and it drops fractions from floats, making them ints;
• this slice indexes[::-3] returns each third item starting from
  the end, so I used another slice [1:] to remove the very last item
  cuz I don’t need a comma after the last number;
• this conditional if l[index] != ‘-‘ is being used to support negative numbers, do not insert a comma after the minus sign.

And a more hardcore version:

def format_number_using_generators_and_list_comprehensions(number):
    string = '%d' % number
    generator = reversed( 
        [
            value+',' if (index!=0 and value!='-' and index%3==0) else value
            for index,value in enumerate(reversed(string))
        ]
    )
    return ''.join(generator)

This is what I do for floats. Although, honestly, I’m not sure which versions it works for – I’m using 2.7:

my_number = 4385893.382939491

my_string = '{:0,.2f}'.format(my_number)

Returns: 4,385,893.38

Update: I recently had an issue with this format (couldn’t tell you the exact reason), but was able to fix it by dropping the 0:

my_string = '{:,.2f}'.format(my_number)

Python 3

—

Integers (without decimal):

"{:,d}".format(1234567)

—

Floats (with decimal):

"{:,.2f}".format(1234567)

where the number before f specifies the number of decimal places.

—

Bonus

Quick-and-dirty starter function for the Indian lakhs/crores numbering system (12,34,567):

https://stackoverflow.com/a/44832241/4928578

I’m surprised that no one has mentioned that you can do this with f-strings in Python 3.6+ as easy as this:

>>> num = 10000000
>>> print(f"{num:,}")
10,000,000

… where the part after the colon is the format specifier. The comma is the separator character you want, so f"{num:_}" uses underscores instead of a comma. Only "," and "_" is possible to use with this method.

This is equivalent of using format(num, ",") for older versions of python 3.

This might look like magic when you see it the first time, but it’s not. It’s just part of the language, and something that’s commonly needed enough to have a shortcut available. To read more about it, have a look at the group subcomponent.

I have a python 2 and python 3 version of this code. I know that the question was asked for python 2 but now (8 years later lol) people will probably be using python 3.

Python 3 Code:

import random
number = str(random.randint(1, 10000000))
comma_placement = 4
print('The original number is: {}. '.format(number))
while True:
    if len(number) % 3 == 0:
        for i in range(0, len(number) // 3 - 1):
            number = number[0:len(number) - comma_placement + 1] + ',' + number[len(number) - comma_placement + 1:]
            comma_placement = comma_placement + 4
    else:
        for i in range(0, len(number) // 3):
            number = number[0:len(number) - comma_placement + 1] + ',' + number[len(number) - comma_placement + 1:]
    break
print('The new and improved number is: {}'.format(number))        

Python 2 Code: (Edit. The python 2 code isn’t working. I am thinking that the syntax is different).

import random
number = str(random.randint(1, 10000000))
comma_placement = 4
print 'The original number is: %s.' % (number)
while True:
    if len(number) % 3 == 0:
        for i in range(0, len(number) // 3 - 1):
            number = number[0:len(number) - comma_placement + 1] + ',' + number[len(number) - comma_placement + 1:]
            comma_placement = comma_placement + 4
    else:
        for i in range(0, len(number) // 3):
            number = number[0:len(number) - comma_placement + 1] + ',' + number[len(number) - comma_placement + 1:]
    break
print 'The new and improved number is: %s.' % (number) 

Slightly expanding the answer of Ian Schneider:

If you want to use a custom thousands separator, the simplest solution is:

'{:,}'.format(value).replace(',', your_custom_thousands_separator)

Examples

'{:,.2f}'.format(123456789.012345).replace(',', ' ')

If you want the German representation like this, it gets a bit more complicated:

('{:,.2f}'.format(123456789.012345)
          .replace(',', ' ')  # 'save' the thousands separators 
          .replace('.', ',')  # dot to comma
          .replace(' ', '.')) # thousand separators to dot

The accepted answer is fine, but I actually prefer format(number,','). Easier for me to interpret and remember.

https://docs.python.org/3/library/functions.html#format

Italy:

>>> import locale
>>> locale.setlocale(locale.LC_ALL,"")
'Italian_Italy.1252'
>>> f"{1000:n}"
'1.000'

this is baked into python per PEP -> https://www.python.org/dev/peps/pep-0378/

just use format(1000, ‘,d’) to show an integer with thousands separator

there are more formats described in the PEP, have at it

Universal solution

I have found some issues with the dot separator in the previous top voted answers. I have designed a universal solution where you can use whatever you want as a thousand separator without modifying the locale. I know it’s not the most elegant solution, but it gets the job done. Feel free to improve it !

def format_integer(number, thousand_separator='.'):
    def reverse(string):
        string = "".join(reversed(string))
        return string

    s = reverse(str(number))
    count = 0
    result = ''
    for char in s:
        count = count + 1
        if count % 3 == 0:
            if len(s) == count:
                result = char + result
            else:
                result = thousand_separator + char + result
        else:
            result = char + result
    return result


print(format_integer(50))
# 50
print(format_integer(500))
# 500
print(format_integer(50000))
# 50.000
print(format_integer(50000000))
# 50.000.000

babel module in python has feature to apply commas depending on the locale provided.

To install babel run the below command.

pip install babel

usage

format_currency(1234567.89, 'USD', locale='en_US')
# Output: $1,234,567.89
format_currency(1234567.89, 'USD', locale='es_CO')
# Output: US$ 1.234.567,89 (raw output US$xa01.234.567,89)
format_currency(1234567.89, 'INR', locale='en_IN')
# Output: ₹12,34,567.89

Here are some ways to do it with formatting (compatible with floats and ints)

num = 2437.68

# Way 1: String Formatting

'{:,}'.format(num)
>>> '2,437.68'


# Way 2: F-Strings

f'{num:,}'
>>> '2,437.68'


# Way 3: Built-in Format Function

format(num, ',')
>>> '2,437.68'

Use separators and decimals together in float numbers :
(In this example, two decimal places)

large_number = 4545454.26262666
print(f"Formatted: {large_number:,.2f}")

Result:
Formatted: 4,545,454.26

Simplest answer:

format (123456, ",")

Result:

'123,456'