title: Pandas groupby quantile values
body: <p>I tried to calculate specific quantile values from a data frame, as shown in the code below. There was no problem when calculate it in separate lines.</p>
  
  <p>When attempting to run last 2 lines, I get the following error: </p>
  
  <pre><code>AttributeError: 'SeriesGroupBy' object has no attribute 'quantile(0.25)'
  </code></pre>
  
  <p>How can I fix this?</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame(
      {
          'x': [0, 1, 0, 1, 0, 1, 0, 1],
          'y': [7, 6, 5, 4, 3, 2, 1, 0],
          'number': [25000, 35000, 45000, 50000, 60000, 70000, 65000, 36000]
      }
  )
  f = {'number': ['median', 'std', 'quantile']}
  df1 = df.groupby('x').agg(f)
  df.groupby('x').quantile(0.25)
  df.groupby('x').quantile(0.75)
  
  # code below with problem:
  f = {'number': ['median', 'std', 'quantile(0.25)', 'quantile(0.75)']}
  df1 = df.groupby('x').agg(f)
  </code></pre>