title: Python: How to add specific columns of .mean to dataframe
body: <p>How can I add the means of b and c to my dataframe? I tried a merge but it didn't seem to work. So I want two extra columns b_mean and c_mean added to my dataframe with the results of <code>df.groupBy('date').mean()</code></p>
  
  <p>DataFrame</p>
  
  <pre><code>  a  b  c  date
  0  2  3  5     1
  1  5  9  1     1
  2  3  7  1     1
  </code></pre>
  
  <p>I have the following code</p>
  
  <pre><code>import pandas as pd
  
  a = [{'date': 1,'a':2, 'b':3, 'c':5}, {'date':1, 'a':5, 'b':9, 'c':1}, {'date':1, 'a':3, 'b':7, 'c':1}]
  
  df = pd.DataFrame(a)
  
  x =  df.groupby('date').mean()
  </code></pre>
  
  <p>Edit:</p>
  
  <p>Desired output would be the following
  <code>df.groupby('date').mean()</code> returns:</p>
  
  <pre><code>             a         b         c
  date                              
  1     3.333333  6.333333  2.333333
  </code></pre>
  
  <p>My desired result would be the following data frame</p>
  
  <pre><code>   a  b  c  date  a_mean   b_mean
  0  2  3  5     1  3.3333   6.3333
  1  5  9  1     1  3.3333   6.3333 
  2  3  7  1     1  3.3333   6.3333
  </code></pre>