title: How can I calculate mean of list of strings？
body: <p>I trying to calculate mean of one colum in a csv file.First, I read one column from <code>.csv</code> file and save it into a list. Next when I try to get mean it have a error</p>
  
  <pre><code>TypeError: 'builtin_function_or_method' object has no attribute '__getitem__'
  </code></pre>
  
  <p>my code is :</p>
  
  <pre><code>with open('XXXXXX.csv') as f:
  reader = csv.DictReader(f) 
  for row in reader: 
      for (k,v) in row.items(): 
          columns_95[k].append(v)
  sVaR5 = columns_95['95%']
  mean_95 = sum（sVaR5）/len(sVaR5)
  </code></pre>
  
  <p>and my csv looks like:</p>
  
  <pre><code>95%      99%
  1.225    2.332
  1.252    10.252
  2.336    4.213
  ...      ...
  </code></pre>
  
  <p>when I check my list, output is<code>['1.225','1.252','2.336']</code> I think maybe the quote mark is the reason why my code has error. but how to fix it!Thanks!!!</p>