title: AttributeError for analyzing multiple sheets in python pandas
body: <p>I included <code>sheet_name=None</code> to analyze every sheet but then I have a problem with reading <code>columns</code>.</p>
  <pre><code>import pandas
  
  df = pd.read_excel('file.xlsx', sheet_name=None, index_col=[0])
  
  df.columns = df.columns.str.split('_', expand=True)
  </code></pre>
  <p>I got this error message</p>
  <pre><code>    df.columns = df.columns.str.split('_', expand=True)
  AttributeError: 'dict' object has no attribute 'columns'
  </code></pre>
  <p>It is working perfectly on one sheet, why won't work for multiple sheets?</p>