title: pandas dataframe to csv, format problem about string of numbers
body: <p>I have a dataframe df, where the first column stores strings like '000001', '000002', '000003'... ect. they are keys of the dataframe with the necessary 0s in front.</p>
  <p>when I try to save df to csv file and call df.to_csv(), the column is saved as 1, 2, 3... instead of the raw data as '000001', '000002'.</p>
  <p>Can I solve this problem when saving data to csv file from pandas dataframe?</p>
  <p>for example:</p>
  <pre><code>df = pd.DataFrame(np.random.normal(0, 1, (3, 3)))
  df.iloc[:, 0] = ['0001', '0002', '0003']
  print(df)
  
  df.to_csv('somedata.csv', index=False)
  
  #then read back or directly open (mine is with excel)
  df = pd.read_csv('somedata.csv')
  print(df)
  </code></pre>
  <p>values in column 0 are 1, 2, 3</p>