title: counting number of customers per week during 6 years
body: <p>I want to figure out number of customers per week during 6 years and add an extra column of number. this is what I tried:</p>
  
  <pre><code>df = df_eind[['year', 'week']].copy()
  countv = df.groupby(['year', 'week']).year.count()
  </code></pre>
  
  <p>and then joined it with the main table which has the customer information like this:</p>
  
  <pre><code>df4 = df_eind.join(countv)
  </code></pre>
  
  <p>but gives an error because can not join a series with a data frame. can you please help me.</p>
  
  <p>the data is like this(ex):</p>
  
  <pre><code> year     week
   2012      2
   2018      50
   2015      16
   2012       2
  </code></pre>
  
  <p>and the file view should be like this(ex):</p>
  
  <pre><code>year  week  count
  2012    2      2
  2015   16      1
  2018   50      1
  </code></pre>