def change(current, previous):
    """Calculate the percent change between `current` and `previous`.
    """
    if current == previous:
        return 0
    try:
        v = ((current - previous) / previous) * 100.0
        if v > 0:
            return "+{:.2f}".format(v)
        return "{:.2f}".format(v)
    except ZeroDivisionError:
        return 0