title: How to filter list of dictionaries with matching values for a given key
body: <p>With an interface like <code>a = copyf(dictlist, key, valuelist)</code>.</p>
  
  <pre><code>&gt;&gt;&gt; dictlist = [{'first': 'James',
                   'last': 'Joule'},
                  {'first': 'James',
                   'last': 'Watt'},
                  {'first': 'Christian',
                   'last': 'Doppler'}]
  &gt;&gt;&gt; valuelist = ['James', 'John']
  &gt;&gt;&gt; x = copyf(dictlist, 'first', valuelist)
  &gt;&gt;&gt; print(x)
  [{'first': 'James',
    'last': 'Joule'},
   {'first': 'James',
    'last': 'Watt'}]
  </code></pre>
  
  <p>The <code>dictlist</code> is effectively a <code>csv.DictReader</code> instance.</p>