Format a number to include commas

This is my attempt to format a number and put commas where appropriate. I was wondering if this could be done in a simpler way.

It's used by calling format_string_number("123456789") and prints 123,456,789.

def format_string_number(st):
    inputVal = list(st)
    inputVal.reverse()
    returnVal = []

    for index, value in enumerate(inputVal):
        returnVal.insert(0,value)
        if (index+1) %3 == 0:
            returnVal.insert(0,",")

    if returnVal[0] == ",":
        returnVal.pop(0)

    print("".join(returnVal))

A few comments:

• It seems odd to write a function for formatting a number that takes a string as an argument;

• Your function doesn't currently deal with float numbers correctly (and, as it takes a string argument, you can't easily check isinstance(arg, int));

• Your function prints the output rather than returning it, severely limiting its usefulness; and

• You aren't compliant with the style guide.

Also, as pointed out in the comments, this is handled by format already, and can be used in str.format too:

>>> "Number: {:,}".format(123456.789)
'Number: 123,456.789'

StackExchange Code Review Q#63398, answer score: 11