def merge(dict1, dict2, func=None):
    """
    update dict1 with key/value pairs in dict2. If they have keys in common,
    merge the respective values.

    :param dict1: dict to be updated
    :param dict2: dict to add to dict1
    :param func: function taking two values and returning a new value.
    :return: None. merge is done in-place.

    >>> d1 = {'roberto': 28, 'chris': 12, 'evan': 121}
    >>> d2 = {'roberto': 3, 'chris': 25, 'epifanio': 350}
    >>> from operator import add
    >>> merge(d1, d2, add)
    >>> d1
    {'chris': 37, 'evan': 121, 'roberto': 31, 'epifanio': 350}
    """
    if func is None:
        dict1.update(dict2)
    else:
        for k in dict2:
            if k in dict1:
                dict1[k] = func(dict1[k], dict2[k])
            else:
                dict1[k] = dict2[k]


def merged(dict1, dict2, func=None):
    """
    Merge two dicts as in merge, and return the merged dict. Does not
    mutate the original dicts.
    :param dict1: dict
    :param dict2: dict
    :param func: merge function  that takes two arguments and returns a value
    :return: merged dict

    >>> d1 = {'robert': 28, 'chrisse': 12, 'evan': 121}
    >>> d2 = {'robert': 3, 'chrisse': 25, 'epifanio': 350}
    >>> from operator import add
    >>> merged(d1, d2, add)
    {'robert': 31, 'evan': 121, 'chrisse': 37, 'epifanio': 350}
    """
    d = dict1.copy()
    if func is None:
        d.update(dict2)
    else:
        for k in dict2:
            if k in d:
                d[k] = func(d[k], dict2[k])
            else:
                d[k] = dict2[k]
    return d

if __name__ == '__main__':
    import doctest
    doctest.testmod()