Format a number to include commas

Question

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))

• Alexis Wilke: I don't know enough python to tell you whether directly dealing with strings would be possible and faster... However, your condition against the index could be written index % 3 == 2 and you'd avoid a +1 on each iteration.
• DSM: Are you looking for a better way to do it manually, or would format(123456789, ",") suffice?
• tiffon: To build off the approach suggested by @DSM : format(int("123456789"), ",")

Answer 1

A few comments:

1. It seems odd to write a function for formatting a number that takes a string as an argument;
2. 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));
3. Your function prints the output rather than returning it, severely limiting its usefulness; and
4. 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'

• user5359531: using , is not locale aware
• jonrsharpe: @user5359531 true! But nor was the original, and that requirement is not in the title or description.
• user5359531: I've made a new question here to address that, perhaps you could check it out.