title: how to arrange pandas dataframe column in descending order
body: <p>I have a pandas dataframe <code>tsp_data_unique</code> like this..</p>
  
  <pre><code>      order_id    quant_bought    Clusters
   0       537            2         Cluster 2
   1       540            1         Cluster 10
   2       541            1         Cluster 4
   3       542            2         Cluster 2
   4       543            2         Cluster 2
   5       545            3         Cluster 2
   6       546            2         Cluster 2
   7       548            3         Cluster 1
   8       549            3         Cluster 6
   9       550            4         Cluster 2
   10      553            1         Cluster 11
   11      555            1         Cluster 1
   12      556            1         Cluster 1
   13      557            1         Cluster 3
   14      584            3         Cluster 2
   15      586            3         Cluster 2
   16      587            3         Cluster 5
   17      588            1         Cluster 7
   18      589            1         Cluster 10
   19      590            1         Cluster 9
   20      591            2         Cluster 2
   21      592            3         Cluster 2
   22      593            1         Cluster 2
   23      594            7         Cluster 8
   24      595            3         Cluster 2
   25      596            1         Cluster 1
   26      608            2         Cluster 2
   27      609            4         Cluster 2
  </code></pre>
  
  <p>And the second dataframe as <code>df</code></p>
  
  <pre><code>      Clusters                                           order_id
  0    Cluster 1                               [548, 555, 556, 596]
  1   Cluster 10                                         [540, 589]
  2   Cluster 11                                              [553]
  3    Cluster 2  [537, 542, 543, 545, 546, 550, 584, 586, 591, ...
  4    Cluster 3                                              [557]
  5    Cluster 4                                              [541]
  6    Cluster 5                                              [587]
  7    Cluster 6                                              [549]
  8    Cluster 7                                              [588]
  9    Cluster 8                                              [594]
  10   Cluster 9                                              [590] 
  </code></pre>
  
  <p>Now I want to arrange the <code>order_id's</code> of <code>Cluster 2</code> of <code>df</code> dataframe <code>Clusters</code> column in descending order by <code>quant_bought</code> column in 1st dataframe.</p>
  
  <p>I am doing following in python.</p>
  
  <pre><code> p  =  df.order_id[df['Clusters'] == 'Cluster 2']
   a = tsp_data_unique['order_id'].isin(p.values[0])
   b = tsp_data_unique['quant_bought'][a]   
   b.sort() 
   b = b[::-1]
   p = tsp_data_unique['order_id'][b.index]
   list_p = p.values.tolist()
   df.order_id[df['Clusters'] == 'Cluster 2'] = list_p
  </code></pre>
  
  <p>When I assign the descending ordered list to <code>Cluster 2</code> in <code>df</code> it gives me an error</p>
  
  <p><code>ValueError: Length of replacements must equal series length</code></p>
  
  <p>Please help..</p>