title: Error with dataframe append
body: <p>I'm trying to append each file's dataframe into a single master dataframe.  The final dataframe is blank, however.  I printed each before trying the append and the independent dataframes have data. </p>
  
  <p>Code:</p>
  
  <pre><code>import pandas as pd
  import os
  
  source_directory = r'H:\folder'
  
  masterDF = pd.DataFrame()
  
  for file in os.listdir(source_directory):
      if file.endswith(".xlsx") or file.endswith(".xls"):
          dataframe = pd.read_excel(source_directory + '\\' + file)
          print(dataframe)
          masterDF.append(dataframe)
  
  print(masterDF)
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>   Col_A  Col_B
  0     46      5
  1     56      4
  2     45      4
  3     45      4
  4    455      5
  5      4      4
  6      4      5
  7    544      4
     Col_A  Col_B
  0     64      9
  1      4     45
  2      4     42
  3     45      4
  4     46      7
  5     56     75
  Empty DataFrame
  Columns: []
  Index: []
  </code></pre>