comma handler for decimal

def comma(n):
    istring = str(n)
    split = istring.split(".")
    if len(split) > 1:
        return comma(int(split[0])) + "." + comma(int(split[1])) #recursive decimal handling
    ostring = ""
    count = 0
    for i in range(len(istring)-1,-1,-1): #for(int i = len(istring); i >= 0; i--)
        if count == 3:
            ostring = "," + ostring
            count = 0
        ostring = istring[i] + ostring
        count += 1
    return ostring