Python question relating to dated values in a csv file
<p>I am loading a data file with dated data into a csv.DictReader</p>

<p>The data looks like this:</p>

<pre><code>date,   weight, blood_pressure, sugar_level
1/1/01, 120.1,  100.1,          25.2
2/1/01, 130.1,  102.1,          26.2
3/1/01, 110.1,  120.1,          24.2
4/1/01, 130.1,  130.1,          28.2
5/1/01, 160.1,  104.1,          27.0
6/1/01, 150.5,  100.1,          22.5
7/1/01, 120.2,  129.1,          25.2
... etc...   
</code></pre>

<p>I want to write a function that allows me to pass in the dictionary (read from the csv file), and return the previous value of a column an epoch ago. Since there are no enumerations in Python, I will use enums to 'constrain' the permissible epochs and column names:</p>

<p>The code snippet I have so far looks something like this:</p>

<pre><code>import csv;

headers = ['date', 'weight', 'blood_pressure', 'sugar_level']
data = csv.DictReader(csvfile, headers);

epoch_type   = (week_ago, fortnight_ago, month_ago);
column_names = headers[1:];

def function get_previous_value(data, column_name, epoch_name):
  """ Note: data is assumed to be sorted in date ascending order  """
  """ Returns the previous value in the data, using the specified """
  """ column name and epoch type                                  """
</code></pre>

<p>I would be grateful if someone could show how to implement this function</p>

<p><strong>[Edit]</strong>
Changed argument name from 'lookback' to 'epoch_name' to aid clarity</p>
