Extract the value corresponding to the given key in all the dictionaries

    >>> bands = [{'name': 'Led Zeppelin', 'singer': 'Robert Plant', 'guitarist': 'Jimmy Page'},
    ... {'name': 'Metallica', 'singer': 'James Hetfield', 'guitarist': 'Kirk Hammet'}]
    >>> valueof(bands, 'singer')
    ['Robert Plant', 'James Hetfield']

def valueof(records, key):
    """Extract the value corresponding to the given key in all the dictionaries

    >>> bands = [{'name': 'Led Zeppelin', 'singer': 'Robert Plant', 'guitarist': 'Jimmy Page'},
    ... {'name': 'Metallica', 'singer': 'James Hetfield', 'guitarist': 'Kirk Hammet'}]
    >>> valueof(bands, 'singer')
    ['Robert Plant', 'James Hetfield']

    """
    if isinstance(records, dict):
        records = [records]
    return map(operator.itemgetter(key), records)