title: python dataframe appending columns horizontally
body: <p>I am trying to make a simple script that concatenates or appends multiple column sets that I pull from xls files within a directory. Each xls file has a format of:</p>
  
  <pre><code>Index    Exp. m/z   Intensity   
  1        1000.11    1000
  2        2000.14    2000
  3        3000.15    3000
  </code></pre>
  
  <p>Each file has varying number of indices. Below is my code:</p>
  
  <pre><code>import pandas as pd
  import os
  import tkinter.filedialog
  
  full_path = tkinter.filedialog.askdirectory(initialdir='.')
  os.chdir(full_path)
  
  data = {}
  df = pd.DataFrame()
  
  for files in os.listdir(full_path):
      if os.path.isfile(os.path.join(full_path, files)):
          df = pd.read_excel(files, 'Sheet1')[['Exp. m/z', 'Intensity']]
          data = df.concat(df, axis=1)
  
  data.to_excel('test.xls', index=False)
  </code></pre>
  
  <p>This produces an attributerror: DataFrame object has no attribute concat. I also tried using append like:</p>
  
  <pre><code>data = df.append(df, axis=1) 
  </code></pre>
  
  <p>but I know that append has no axis keyword argument. df.append(df) does work, but it places the columns at the bottom. I want something like:</p>
  
  <pre><code>Exp. m/z   Intensity       Exp. m/z   Intensity  
  1000.11    1000            1001.43    1000
  2000.14    2000            1011.45    2000
  3000.15    3000
  </code></pre>
  
  <p>and so on. So the column sets that I pull from each file should be placed to the right of the previous column sets, with a column space in between. </p>