title: Best way to test if large file exists
body: <p>I would like to know what is most efficient way to test if a large file exists locally (without loading it in memory). If it doesn't exists (or not readable) then download it. The goal is to upload the data in a pandas DataFrame.</p>
  
  <p>I wrote the snippet below which is working (and tested with a small file). What about correctness and pythonic programming?</p>
  
  <pre><code>url = "http://www-bcf.usc.edu/~gareth/ISL/Advertising.csv" # 4.7kB  
  file = "./test_file.csv" 
  
  try:
      os.open( file, os.O_RDONLY)
      df_data = pd.read_csv( file, index_col=0)
  
  except: 
      df_data = pd.read_csv( url, index_col=0)
      df_data.to_csv( file)
  </code></pre>