title: TypeError: unhashable type: 'list' when use groupby in python
body: <p>There is something wrong when I use groupby method:</p>
  
  <pre><code>data = pd.Series(np.random.randn(100),index=pd.date_range('01/01/2001',periods=100))
  keys = lambda x: [x.year,x.month]
  data.groupby(keys).mean()
  </code></pre>
  
  <p>but it has an error: TypeError: unhashable type: 'list'.
  I want group by year and month, then calculate the means,why it has wrong?</p>