title: I want to add columns to a pandas data frame using data fromother columns
body: <p>I am importing covid data from a web site into a pandas dataframe.  The data included cases and deaths. I would like to add columns for cum_cases and cum_deaths. This is simple in a spread sheet and I would do it before importing the csv file with the data but there are too many rows to make it easy and besides I would like to know how.  With a spread sheet I insert a 0 into the first cell of each the cumulative cases (cum_cases) and cumulative deaths (cum_deaths) columns then use formulas - for example if I was in cell d5  the formula would be d4+c5 assuming the two columns for cases or deaths were adjacent.</p>
  
  <p>But I don't have a clue how to do the same thing in a dataframe and have spent several days searching duckduckgo and google for solutions to no avail.</p>
  
  <p>What I have</p>
  
  <pre><code>|Date        |cases  |deaths  |
  |:----------:|:-----:|:------:|
  |2019-12-31  |  0    |  0     |
  |2020-01-01  |  1    |  2     |
  |2020-01-02  |  3    |  5     |
  
  </code></pre>
  
  <p>What I want</p>
  
  <pre><code>
  |Date        |cases  |deaths  |cum_cases| cum_deaths|
  |:----------:|:-----:|:------:|:-------:|:---------:|
  |2019-12-31  |  0    |  0     |0        | 0         |
  |2020-01-01  |  1    |  2     |1        | 2         |
  |2020-01-02  |  3    |  5     |4        | 7         |
  |2020-01-03  |  1    |  2     |5        | 9         |
  
  </code></pre>