title: How to append dataframe to an empty dataframe using concurrent
body: <p>I want to run a function using <code>concurrent</code> in Python. This is the function that I have :</p>
  
  <pre><code>import concurrent.futures
  import pandas as pd
  import time
  
  def putIndf(file):
      listSel = getline(file)
      datFram = savetoDataFrame(listSel)
      return datFram #datatype : dataframe
  
  def main():
      newData = pd.DataFrame()
      with concurrent.futures.ProcessPoolExecutor(max_workers=30) as executor:
          for i,file in zip(fileList, executor.map(dp.putIndf, fileList)):
              df = newData.append(file, ignore_index=True)
      return df
  
  if __name__ == '__main__':
      main()
  </code></pre>
  
  <p>I want to join dataframe to be one dataframe <code>newData</code>, but the result is only the last dataframe from that function</p>