title: Pandas dataframe concatenation in for loop resulting in empty dataframes
body: <p>I am sure that I am missing something obvious. The concatenation works because "print d" prints exactly what I want, but for some reason it doesn't stick outside of the for loop. If I ask to print one of the dataframes outside the loop, they are all empty dataframes. Please help.</p>
  
  <pre><code>file_name = ['anal_cc_cbs/ld-non_aug-atqd', 'anal_cc_cbs/ld-non_aug-tqt', 'anal_cc_cbs/ld-non_aug-q5t', 
          'anal_cc_cbs/ld-atqd', 'anal_cc_cbs/ld-tqt', 'anal_cc_cbs/ld-q5t', 'anal_fnocc_cbs/ld-non-aug-atqd', 
          'anal_fnocc_cbs/ld-non-aug-tqt', 'anal_fnocc_cbs/ld-non-aug-q5t', 'anal_fnocc_cbs/ld-atqd', 
          'anal_fnocc_cbs/ld-tqt', 'anal_fnocc_cbs/ld-q5t', 'anal_fnocc_cbs/ld-56t', 'vdz_rad', 'vtz_rad', 'vqz_rad',
          'v5z_rad', 'avdz_rad', 'avtz_rad', 'avqz_rad', 'av5z_rad', 'vdz-f12_rad', 'vtz-f12_rad', 'tzvpp_rad',
          'qzvpp_rad']
  
  o_tqd_df=pd.DataFrame(); o_tqt_df=pd.DataFrame(); o_q5t_df=pd.DataFrame(); 
  o_atqd_df=pd.DataFrame(); 
  o_atqt_df=pd.DataFrame(); o_aq5t_df=pd.DataFrame(); o_ftqd_df=pd.DataFrame(); 
  o_ftqt_df=pd.DataFrame(); 
  o_fq5t_df=pd.DataFrame(); o_fatqd_df=pd.DataFrame(); 
  o_fatqt_df=pd.DataFrame(); o_faq5t_df=pd.DataFrame(); 
  o_fa56t_df=pd.DataFrame(); o_vdz_df=pd.DataFrame(); o_vtz_df=pd.DataFrame(); 
  o_vqz_df=pd.DataFrame(); 
  o_v5z_df=pd.DataFrame(); o_avdz_df=pd.DataFrame(); o_avtz_df=pd.DataFrame(); 
  o_avqz_df=pd.DataFrame();
  o_av5z_df=pd.DataFrame(); o_vdz_f12_df=pd.DataFrame(); 
  o_vtz_f12_df=pd.DataFrame(); o_tzvpp_df=pd.DataFrame(); 
  o_qzvpp_df=pd.DataFrame() 
  
  df_name = [o_tqd_df, o_tqt_df, o_q5t_df, o_atqd_df, o_atqt_df, o_aq5t_df, 
  o_ftqd_df, o_ftqt_df, o_fq5t_df, o_fatqd_df,
        o_fatqt_df, o_faq5t_df, o_fa56t_df, o_vdz_df, o_vtz_df, o_vqz_df, 
  o_v5z_df, o_avdz_df, o_avtz_df, o_avqz_df,
        o_av5z_df, o_vdz_f12_df, o_vtz_f12_df, o_tzvpp_df, o_qzvpp_df]
  
  for f, d in zip(file_name, df_name):
      allFiles = glob.glob('/theoryfs2/ds/warden/projects/fp_opt/' + f + '/ld-*')
      dlist = []
      for fil in allFiles:
          df = pd.read_csv(fil)
          dlist.append(df)
      d = pd.concat(dlist)
      print d
  </code></pre>