title: how to calculate variance and std without import
body: <p>I have to calculate the mean,variance and the std in python.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>def mean(first_data_values):
  
      return sum(first_data_values) / len(first_data_values)
  
  def variance(values):
  
      meanval = mean(values)
      return sum((values[i]-meanval) ** 2 for i in values)/len(values)
  
  def std(values):
  
      return (variance(values)) ** (1/2)
  </code></pre>
  
  <p>So i get the wrong result. Any idea what`s wrong??</p>