def describe_builtin(obj):
    """ Describe a builtin function if obj.__doc__
    available.

    Parameters
    ----------
    obj : python object
    
    Returns
    -------
    iterator : builtin args
    """
    docstr = obj.__doc__
    args = ''
    if docstr:
        items = docstr.split('\n')
        if items:
            func_descr = items[0]
            s = func_descr.replace(obj.__name__, '')
            idx1 = s.find('(')
            idx2 = s.find(')', idx1)
            if idx1 != -1 and idx2 != -1 and idx2 > idx1 + 1:
                args = s[idx1 + 1:idx2]
                for arg in args:
                    yield arg
    if args == '':
        yield None
