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)
    returnVal = []

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

    if returnVal[0] == ",":

  • \$\begingroup\$ 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. \$\endgroup\$ – Alexis Wilke Sep 20 '14 at 2:48
  • \$\begingroup\$ Are you looking for a better way to do it manually, or would format(123456789, ",") suffice? \$\endgroup\$ – DSM Sep 20 '14 at 5:34
  • \$\begingroup\$ To build off the approach suggested by @DSM : format(int("123456789"), ",") \$\endgroup\$ – tiffon Sep 28 '14 at 7:03

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'
  • \$\begingroup\$ using , is not locale aware \$\endgroup\$ – user5359531 Jan 2 at 22:24
  • \$\begingroup\$ @user5359531 true! But nor was the original, and that requirement is not in the title or description. \$\endgroup\$ – jonrsharpe Jan 2 at 22:32
  • \$\begingroup\$ I've made a new question here to address that, perhaps you could check it out. \$\endgroup\$ – user5359531 Jan 2 at 23:07

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