title: Unable to append file in python
body: <p>I have this script which is to pull stock prices for the last so many years from yahoo finance. </p>
  
  <p>Once the code is run, i am not able to write again on the file even though it is suppose to check what exists and append. </p>
  
  <p>I get the following error:
  FileExistsError: [Errno 17] File exists: 'stocks_dfs'</p>
  
  <p>Please advise!</p>
  
  <p><code> 
  '''python
      import bs4 as bs
      import pickle
      import requests
      import datetime as dt
      import pandas as pd
      import os
      import pandas_datareader.data as web
      from time import sleep</p>
  
  <pre><code>def save_sp500_tickers():
      resp = requests.get('https://en.wikipedia.org/wiki/List_of_S%26P_500_companies')
      soup =bs.BeautifulSoup(resp.text,'lxml')
      table=soup.find('table',{'class':'wikitable sortable'})
      tickers=[]
      for row in table.findAll('tr')[1:]:
          ticker= row.findAll('td')[1].text
          tickers.append(ticker)
  
      with open("sp500ticker.pickle", "wb") as f:
          pickle.dump(tickers,f)
  
      print (tickers)
  
      return tickers
  #save_sp500_tickers()
  
  def get_data_from_yahoo(reload_sp500=False):
      if reload_sp500:
          tickers=save_sp500_tickers()
      else:
          with open("sp500ticker.pickle","rb") as f:
              tickers=pickle.load(f)
      if not os.path.exists('stock_dfs'):
          os.makedirs('stocks_dfs')
  
      start = dt.datetime(2016,1,1)
      end = dt.datetime.now()
  
      for ticker in tickers:
          print(ticker)
  
          if not os.path.exists('stocks_dfs/{}.csv'.format(ticker)):
              df = web.DataReader(ticker,'yahoo', start, end)
              df.to_csv('stocks_dfs/{}.csv'.format(ticker))
          else:
              print('Already have {}'.format(ticker))
  
  get_data_from_yahoo()
  </code></pre>
  
  <p>'''
  </code></p>