Howw to prevent the duplication of any value in a column within a CSV file (python)

[Adam Davis]

On Tuesday, 26 April 2016 17:14:36 UTC+1, Ian  wrote:
> On Tue, Apr 26, 2016 at 10:01 AM,  <peakgraphicz at gmail.com> wrote:
> > I am wondering how to make my code function so it does not allow any of the same values to be entered into a column in my CSV file created through python. So I need to read into the CSV file and check if any names have already been entered. If they have, the user must not be allowed to enter this value.
> 
> Assuming the file is reasonably small enough to fit into memory,
> create a set and populate it with the values in the column. When the
> user enters a new value, check whether it's already in the set.

That's exactly what I want to do, however, I'm unsure how to check whether it's in the set?