title: How to append new dataframe rows to a csv using pandas?
body: <p>I have a new dataframe, how to append it to an existed csv? </p>
  
  <p>I tried the following code:</p>
  
  <pre><code>f = open('test.csv', 'w')
  df.to_csv(f, sep='\t')
  f.close()
  </code></pre>
  
  <p>But it doesn't append anything to test.csv. The csv is big, I only want to use append, rather than read the whole csv as dataframe and concatenate it to and write it to a new csv. Is there any good method to solve the problem? Thanks.</p>