title: Sample each group after pandas groupby
body: <p>I know this must have been answered some where but I just could not find it.</p>
  
  <p><strong>Problem</strong>: Sample each group after groupby operation.</p>
  
  <pre class="lang-py prettyprint-override"><code>import pandas as pd
  
  df = pd.DataFrame({'a': [1,2,3,4,5,6,7],
                     'b': [1,1,1,0,0,0,0]})
  
  grouped = df.groupby('b')
  
  # now sample from each group, e.g., I want 30% of each group
  </code></pre>