title: python combine dataframe data with same index
body: <p>I have a data frame with an index(date) and a numerical column. It is currently in the below format</p>
  <pre><code>2022-01-01  52
  2022-01-01  12
  2022-01-02   5
  2022-01-02  -3
  2022-01-03  10
  </code></pre>
  <p>I want to combine all the data that has the same index date. The desired end result should be</p>
  <pre><code>2022-01-01  64
  2022-01-02   2
  2022-01-03  10
  </code></pre>