use the `locale.currency` library function

I had stupidly written a pair of functions like this:

def comma_sep(amount):
    thousands = int(amount/1000)
    remainder = int(amount%1000)
    if thousands > 0:
        return comma_sep(thousands)+","+"%.3d"%(remainder)
    else:
        return str(remainder)
def friendly_from_cents(total_cents):
    pos = total_cents >= 0
    total_cents = abs(total_cents)
    cents = int(total_cents%100)
    dollars = int(total_cents/100)
    friendly = "$"+comma_sep(dollars)+"."+str(cents)
    if pos:
        return friendly
    else:
        return "-" + friendly

This is stupid, because python has builtin support to localize currency:

import locale
locale.setlocale(locale.LC_ALL, 'en_US')
locale.currency(-1800, grouping=True)

I think it’s tempting to fall into these traps, because it always starts simpler: I had some total_cents variable and just wanted to display quick and dirty. The complexity built up gradually, until I had something pretty complicated. Oops.

More generally, I violated the “Use the batteries” idiom of Python – which is something I actually try to do. Kinda embarrassing.