title: Python Pandas - Adding new sum to every row
body: <p>I'm new to python coding and I'm struggling to add a new column to my pandas framework.</p>
  
  <p>Here's what the column looks like:
  Time_Difference (it's in seconds)
  3
  2
  5
  6</p>
  
  <p>It's a float64 type.
  And what I wanna create is the following new column :
  Duration
  3
  5
  10
  16</p>
  
  <p>Actually in the format 00:00:00 (The duration adds up to about 2h since I'm tracking a run.)</p>
  
  <p>Now, how to create a new column, that's clear but I struggle with the itterows() or apply() function.</p>
  
  <p>In my naive perception, it should look something like this:
  df["Duration"] = df["Time_Difference"] + df["Time_Difference"].shift(1)
  Well, obviously it doesn't.</p>
  
  <p>Couldn't find anything similar on stackoverflow as I don't wanna have the sum of the whole column but on every row.</p>