title: Get list of indices where key-value doens't match both dataframes
body: <p>I have two dataframes.</p>
  <pre><code>left = pd.DataFrame(
      {
          &quot;key&quot;: [&quot;K0&quot;, &quot;K1&quot;, &quot;K2&quot;, &quot;K3&quot;],
          &quot;A&quot;: [&quot;A0&quot;, &quot;A1&quot;, &quot;A2&quot;, &quot;A3&quot;],
          &quot;B&quot;: [&quot;B0&quot;, &quot;B1&quot;, &quot;B2&quot;, &quot;B3&quot;],
      }
  )
  
  
  right = pd.DataFrame(
      {
          &quot;key&quot;: [&quot;K0&quot;, &quot;K1&quot;, &quot;K2&quot;, &quot;K3&quot;, &quot;K4&quot;, &quot;K5&quot;],
          &quot;C&quot;: [&quot;C0&quot;, &quot;C1&quot;, &quot;C2&quot;, &quot;C3&quot;, &quot;C4&quot;, &quot;C5&quot;],
          &quot;D&quot;: [&quot;D0&quot;, &quot;D1&quot;, &quot;D2&quot;, &quot;D3&quot;, &quot;D4&quot;, &quot;D5&quot;],
      }
  )
  </code></pre>
  <p>The <code>left</code> df is part of the <code>right</code> df, as you can see:</p>
  <pre><code>left = 
      key A   B
  0   K0  A0  B0
  1   K1  A1  B1
  2   K2  A2  B2
  3   K3  A3  B3
  
  right = 
      key C   D
  0   K0  C0  D0
  1   K1  C1  D1
  2   K2  C2  D2
  3   K3  C3  D3
  4   K4  C4  D4
  5   K5  C5  D5
  </code></pre>
  <p>Based on the <code>key</code> there is a difference in <code>K4</code> and <code>K5</code> between both dataframes.</p>
  <p>My goal is to get a list of indices who are NOT in both dataframes, e.g. <code>[4, 5]</code> based on the key-value.</p>