title: Pandas .loc without KeyError
body: <pre><code>&gt;&gt;&gt; pd.DataFrame([1], index=['1']).loc['2']  # KeyError
  &gt;&gt;&gt; pd.DataFrame([1], index=['1']).loc[['2']]  # KeyError
  &gt;&gt;&gt; pd.DataFrame([1], index=['1']).loc[['1','2']]  # Succeeds, as in the answer below. 
  </code></pre>
  
  <p>I'd like something that doesn't fail in either of</p>
  
  <pre><code>&gt;&gt;&gt; pd.DataFrame([1], index=['1']).loc['2']  # KeyError
  &gt;&gt;&gt; pd.DataFrame([1], index=['1']).loc[['2']]  # KeyError
  </code></pre>
  
  <p>Is there a function like <code>loc</code> which gracefully handles this, or some other way of expressing this query? </p>