title: calculationg the mean from each Dataframe column
body: <p>I've to write a function (column_means), that calculates the <strong>mean of each column</strong> from Dataframe and give me a list of means at the end. I'm not allowed to use the mean function <strong>.mean()</strong>, so I'm implementing the general formula of the mean: sum(x_i)/Number of elements.</p>
  <p>This is my code:</p>
  <pre><code>df = pd.DataFrame({'a':[1,2,3], 'b': [4,5,6]})
  def column_means(df):
      means  = [] 
      for i,n in zip(df.columns,  df.shape[0]):
          means [n] = sum(df[i])/ df.shape[0]
      return means
  </code></pre>
  <p>It doesn't work as intended. could you please help me and tell me, what are my mistakes?</p>
  <p>Thank you in advance.</p>