Format a number to include commas
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:

&gt;&gt;&gt; "Number: {:,}".format(123456.789)
'Number: 123,456.789'