title: UserWarning from Python pandas about reindexed Boolean Series key
body: <p>When I use indexes to visit a pandas.DataFrame, it gave a userswarnings and they didn't interfere its output. I expect to know how did this userwarning happen and what should I do to avoid these userwarning? Thanks for everyone's attention.</p>
  
  <pre><code>df = pandas.DataFrame([[k, ass4Dict[k], k[:2], k[-2:]] for k in ass4Dict])
  df.columns = ['string', 'count', 'lstr', 'rstr']
  df = df[df['count'] &gt;= 10]
  **df = df[df['lstr'].map(lambda x:x in gram2Dict)][df['rstr'].map(lambda x:x in gram2Dict)]**
  df['lstrCNT'] = df['lstr'].map(lambda x: float(gram2Dict[x]))
  df['rstrCNT'] = df['rstr'].map(lambda x: float(gram2Dict[x]))
  df['conPow'] = df['lstrCNT'] * df['rstrCNT']
  df['lstrPow'] = df['count'] / df['lstrCNT']
  df['rstrPow'] = df['count'] / df['rstrCNT']
  df['aux4Ratio'] = df['count'] / df['conPow']
  df['aux4Log'] = df['aux4Ratio'].map(lambda x: -log(x))
  **df = df[df['aux4Log'] &lt; 11][df['lstrPow'] &gt;= 0.5][df['rstrPow'] &gt;= 0.5]**
  </code></pre>
  
  <p>....</p>
  
  <p>沈钦言 359</p>
  
  <p>纪小蕊 158</p>
  
  <p>顾持钧 949</p>
  
  <p>林晋修 642</p>
  
  <p>4</p>
  
  <p>0.256721019745 1.22976207733</p>
  
  <p>ch_extract.py:153: UserWarning: Boolean Series key will be reindexed to match DataFrame index.
    df = df[df['lstr'].map(lambda x:x in gram2Dict)][df['rstr'].map(lambda x:x in gram2Dict)]
  ch_extract.py:161: UserWarning: Boolean Series key will be reindexed to match DataFrame index.
    df = df[df['aux4Log'] &lt; 11][df['lstrPow'] >= 0.5][df['rstrPow'] >= 0.5]</p>