Python Query list for oldest date
<p>My query returns the following in a list:</p>

<pre><code>"Alex";"275467125";"2015-02-03 02:55:36-05";"1"
"Alex";"275467125";"2015-01-13 02:09:39-05";"1"
"Alex";"275467125";"2015-01-05 04:13:35-05";"1"
"Alex";"275467125";"2014-12-27 04:55:47-05";"1"
"Alex";"275467125";"2014-12-27 04:54:52-05";"1"
"Alex";"275467125";"2014-12-07 03:13:24-05";"1"
"Alex";"275467125";"2014-12-04 03:34:56-05";"1"
"Alex";"275467125";"2014-12-02 04:16:33-05";"1"
"Ali";"275464747";"2016-02-17 10:52:12-05";"2"
"Alladin";"275467455";"2016-03-13 06:51:52-04";"2"
"Alladin";"275467455";"2016-03-13 06:51:47-04";"2"
"Anna";"275467401";"2016-03-26 03:56:41-04";"1"
"Anna";"275467401";"2016-03-26 03:55:21-04";"1"
"Anna";"275467401";"2016-03-21 23:04:28-04";"1"
"Anna";"275467401";"2016-02-12 13:24:44-05";"1"
"Anna";"275467401";"2015-12-03 08:20:35-05";"1"
"Anna";"275467401";"2015-11-09 04:18:27-05";"1"
"Anna";"275467401";"2015-11-09 04:11:59-05";"1"
"Anna";"275467401";"2015-09-13 21:27:12-04";"1"
</code></pre>

<p>I want to create a dictionary of person's name with the oldest record they have. I've figured out:</p>

<pre><code>oldestlist = {d[0]:d[2] for d in records}
</code></pre>

<p>This returns a correct answer but my worry is that if I am presented a list that is not formatted in a descending order of date/time it will not provide the correct answer. What is the best way to create a dictionary with a name and the oldest date?</p>

<p>You don't need to sort any data, just use a <em>defaultdict</em> and check the current date vs any new date and update accordingly:</p>

<pre><code>s = """"Alex";"275467125";"2015-02-03 02:55:36-05";"1"
"Alex";"275467125";"2015-01-13 02:09:39-05";"1"
"Alex";"275467125";"2015-01-05 04:13:35-05";"1"
"Alex";"275467125";"2014-12-27 04:55:47-05";"1"
"Alex";"275467125";"2014-12-27 04:54:52-05";"1"
"Alex";"275467125";"2014-12-07 03:13:24-05";"1"
"Alex";"275467125";"2014-12-04 03:34:56-05";"1"
"Alex";"275467125";"2014-12-02 04:16:33-05";"1"
"Ali";"275464747";"2016-02-17 10:52:12-05";"2"
"Alladin";"275467455";"2016-03-13 06:51:52-04";"2"
"Alladin";"275467455";"2016-03-13 06:51:47-04";"2"
"Anna";"275467401";"2016-03-26 03:56:41-04";"1"
"Anna";"275467401";"2016-03-26 03:55:21-04";"1"
"Anna";"275467401";"2016-03-21 23:04:28-04";"1"
"Anna";"275467401";"2016-02-12 13:24:44-05";"1"
"Anna";"275467401";"2015-12-03 08:20:35-05";"1"
"Anna";"275467401";"2015-11-09 04:18:27-05";"1"
"Anna";"275467401";"2015-11-09 04:11:59-05";"1"
"Anna";"275467401";"2015-09-13 21:27:12-04";"1"
"""

import  csv
from collections import defaultdict

d = defaultdict(str)
for name,_, date, _ in csv.reader(s.splitlines(), delimiter=";"):
    if not d[name] or d[name] &gt; date:
        d[name] = date


from pprint import pprint as pp

pp(dict(d))
</code></pre>

<p>Output:</p>

<pre><code> {'Alex': '2014-12-02 04:16:33-05',
 'Ali': '2016-02-17 10:52:12-05',
'Alladin': '2016-03-13 06:51:47-04',
'Anna': '2015-09-13 21:27:12-04'}
</code></pre>

<p>because the dates are in the <em>y-m-d time</em> format it is safe to do a  lexicographical comparison.</p>

python list date dictionary time