!~~digit
        '0123456789'.isdigit() or 123456789

def digit(nstr, schema):
    """
    !~~digit
        '0123456789'.isdigit() or 123456789
    """
    if isinstance(nstr, int):
        nstr = str(nstr)
    elif not isinstance(nstr, basestring):
        return False

    return nstr.isdigit()