title: DataFrame won't assign values to new pandas column
body: <p>I am trying to add all values from pandas DataFrames <code>df1.Number</code> to <code>df2.Number</code> where the date ranges match.
  I have a created <code>df1</code> which looks as such:</p>
  
  <pre><code>       Start        End  Number
  0 2000-01-01 2000-01-08       1
  1 2000-01-02 2000-01-09       2
  2 2000-01-03 2000-01-10       3
  3 2000-01-04 2000-01-11       4
  4 2000-01-05 2000-01-12       5
  5 2000-01-06 2000-01-13       6
  6 2000-01-07 2000-01-14       7
  </code></pre>
  
  <p>and df2 which looks as such:</p>
  
  <pre><code>        Dates Number
  0  2000-01-01       
  1  2000-01-02       
  2  2000-01-03       
  3  2000-01-04       
  4  2000-01-05       
  5  2000-01-06       
  6  2000-01-07       
  7  2000-01-08       
  8  2000-01-09       
  9  2000-01-10       
  10 2000-01-11       
  11 2000-01-12       
  12 2000-01-13       
  13 2000-01-14     
  </code></pre>
  
  <p>and I am trying to assign <code>Number</code> from <code>df1</code> to <code>Number</code> in <code>df2</code> if the date range in the rows match</p>
  
  <p>The code I have used is for the first row is:</p>
  
  <pre><code>i = pd.date_range(df1.Start[0], df1.End[0])
  
  if df2.Dates in i:
      df2.Number.assign(df1.Number)
  </code></pre>
  
  <p>which does not throw an error but also does not assign anything to <code>Number</code>. Expected output should be as follows:</p>
  
  <pre><code>        Dates Number
  0  2000-01-01 1     
  1  2000-01-02 1,2      
  2  2000-01-03 1,2,3    
  3  2000-01-04 1,2,3,4 
  4  2000-01-05 1,2,3,4,5
  5  2000-01-06 1,2,3,4,5,6      
  6  2000-01-07 1,2,3,4,5,6,7    
  7  2000-01-08 1,2,3,4,5,6,7   
  8  2000-01-09 2,3,4,5,6,7
  9  2000-01-10 3,4,5,6,7
  10 2000-01-11 4,5,6,7
  11 2000-01-12 5,6,7
  12 2000-01-13 6,7 
  13 2000-01-14 7 
  </code></pre>
  
  <p>Can anyone suggest why nothing is being assigned or a code that would do this for all rows? Thanks.</p>