Formats a number field.

def format_number_field(__, prec, number, locale):
    """Formats a number field."""
    prec = NUMBER_DECIMAL_DIGITS if prec is None else int(prec)
    locale = Locale.parse(locale)
    pattern = locale.decimal_formats.get(None)
    return pattern.apply(number, locale, force_frac=(prec, prec))