Prints out price stats for cards available in Steam user inventory.

def get_cards_stats(ctx, currency, skip_owned, appid, foil):
    """Prints out price stats for cards available in Steam user inventory."""

    username = ctx.obj['username']
    cards_by_app = defaultdict(list)

    inventory = User(username).traverse_inventory(item_filter=TAG_ITEM_CLASS_CARD)
    for item in inventory:
        appid_ = item.app.appid
        if not appid or appid_ in appid:
            cards_by_app[appid_].append(item)

    if not cards_by_app:
        click.secho('User `%s` has no cards' % username, fg='red', err=True)
        return

    for appid_, cards in cards_by_app.items():
        app = cards[0].app
        print_card_prices(
            app.appid, currency,
            owned_cards=[card.title for card in cards],
            skip_owned=skip_owned,
            foil=foil,
        )