title: Combine Pandas data frame column values into new column
body: <p>I'm working with Pandas and I have a data frame where we can have one of three values populated:</p>
  
  <pre><code>ID_1    ID_2    ID_3
  abc     NaN     NaN
  NaN     def     NaN
  NaN     NaN     ghi
  NaN     NaN     jkl
  NaN     mno     NaN
  pqr     NaN     NaN
  </code></pre>
  
  <p>And my goal is to combine these three columns into a new columns in my data frame:</p>
  
  <pre><code>ID_1    ID_2    ID_3  Combined_ID
  abc     NaN     NaN    abc
  NaN     def     NaN    def
  NaN     NaN     ghi    ghi
  NaN     NaN     jkl    jkl
  NaN     mno     NaN    mno
  pqr     NaN     NaN    pqr
  </code></pre>
  
  <p>Ideally it would just find whatever not null value exists in columns 1 through 3, but I could also concatenate since we should only have one of the three populated for each row. Thanks.</p>
  
  <pre><code>df_note = pd.read_csv("NoteIds.csv")
  df_note['Combined_ID'] = # ID_1 + ID_2 + ID_3
  </code></pre>