title: Delete rows from csv containing non-numeric content at a specific column
body: <p>Here is my test data (in csv format):</p>
  
  <pre><code>"id","user_id","value"
  "1","1001","111"
  "2","1002","222"
  "3","h82kn9;2&amp;3","333"
  "4","jsidofj882","444"
  </code></pre>
  
  <p>The goal is to eliminate all rows with a string as <code>user_id</code>. I would like to know how I can test whether the content of each line, corresponding to a certain column, is or is not a string. It could be <em>any</em> string.</p>
  
  <p>I have made baby steps at writing the code, but I have to admit I have no idea how to proceed for the <code>if</code> part.</p>
  
  <pre><code>import csv
  reader = csv.reader(open("file.csv", "rb"), delimiter=",", quotechar='"')
  for line in reader:
      if ##column 'user_id' is not a string##
          print line
  </code></pre>
  
  <p>Any hints would be appreciated.</p>