title: Changing column values for a value in an adjacent column in the same dataframe using Python
body: <p>I am quite new to Python programming.
  I am working with the following dataframe:</p>
  <p><a href="https://i.stack.imgur.com/uNy96.png" rel="nofollow noreferrer">Before</a></p>
  <p>Note that in column &quot;FBgn&quot;, there is a mix of FBgn and FBtr string values. I would like to replace the FBtr-containing values with FBgn values provided in the adjacent column called &quot;## FlyBase_FBgn&quot;. However, I want to keep the FBgn values in column &quot;FBgn&quot;. Maybe keep in mind that I am showing only a portion of the dataframe (reality: 1432 rows). How would I do that? I tried the replace() method from Pandas, but it did not work.
  This is actually what I would like to have:</p>
  <p><a href="https://i.stack.imgur.com/mJh1N.png" rel="nofollow noreferrer">After</a></p>
  <p>Thanks a lot!</p>