return a concatenation of dictionaries ``a`` and ``b`` prefering values of b but extending lists

gistfile1.py

def smart_update(a, b):
    """
    return a concatenation of dictionaries ``a`` and ``b``
    prefering values of b but extending lists

    >>> a = {'name': 'Martin', 'things': [1]}
    >>> b = {'name': 'Nati', 'things': [2]}
    >>> smart_update(a, b)
        {'name': 'Nati', 'cosas': [1, 2]}

    """
    r = a.copy()
    r.update(b)
    for (k, v) in a.items():
        if isinstance(v, list) and isinstance(b.get(k, None), list):
            r[k] = v + b[k]
    return r