title: Concatenating two dfs in Pandas
body: <p>I wanted to know if there was a way to verify if a value in one df that was being Concatenated with another existed in both dfs or the other df as well prior to stacking them on top of each other??</p>
  
  <p>I had two dfs as</p>
  
  <pre><code>df = pd.concat([df1, df2])
  df = df.sort_values(by=['id', 'timestamp']).reset_index(drop=True)
  df
  
     id  timestamp
  0   1 1959-06-01
  1   1 2019-01-01
  2   1 2019-01-02
  3   2 1989-12-01
  4   2 2019-01-15
  5   3 1999-01-25
  6   3 2019-01-17
  7   3 2019-02-01
  8   3 2019-02-03
  </code></pre>
  
  <p>Was there a way to to verify that the I.D in df1 existed in df2 before concatenating similar to a merge? I didn't need to merge but instead concatenate the dfs on top of each other.</p>
  
  <p>One df has multiple ids and timestamps and the other had only one ID and I wanted to make sure only IDs that existed in both were in the resulting concatenated df</p>
  
  <p>thanks!</p>