title: Python - lookup a value from another df multiple times
body: <p>I have the following two dataframes:</p>
  <pre><code>   prod_id       land_ids
  0  1             [1,2]
  1  2             [1]
  2  3             [2,3,4]
  3  4             []
  4  5             [3,4]
  
     land_id       land_desc
  0  1             germany
  1  2             austria
  2  3             switzerland
  3  4             italy
  </code></pre>
  <p>Bascially, I want all numbers in column land_ids to individually join the other df.</p>
  <p>The result should look something like this:</p>
  <pre><code>   prod_id       land_ids  list_land
  0  1             [1,2]     germany austria
  1  2             [1]       germany
  2  3             [2,3,4]   austria switzerland italy
  3  4             []     
  4  5             [3,4]     switzerland italy
  </code></pre>
  <p>Preferrably, the column list_land is one string where the lands are concatenated. But I would also be fine with getting a list as a result.</p>
  <p>Any idea on how to do this?</p>
  <p>Here is my code for creating the df:</p>
  <pre><code>data_prod = {'prod_id': [1,2,3,4,5], 'land_ids': [[1,2],[1],[2,3,4],[1,3],[3,4]]}
  prod_df = pd.DataFrame(data_prod)
  
  data_land = {'land_id': [1,2,3,4], 'land_desc': ['germany', 'austria', 'switzerland', 'italy']}
  land_df = pd.DataFrame(data_land)
  </code></pre>
  <p>EDIT: what do I have to add if one value of land_ids is empty?</p>