pandas grouper int by frequency
<p>I would like to group a Pandas dataframe by hour disregarding the date.</p>

<p>My data:</p>

<pre><code>id    opened_at             count   sum
      2016-07-01 07:02:05   1       46.14
154   2016-07-01 07:34:02   1       479
      2016-07-01 10:10:01   1       127.14
      2016-07-02 12:01:04   1       8.14
      2016-07-02 12:00:50   1       18.14
</code></pre>

<p>I am able to group by hour with date taken into account by using the following.</p>

<pre><code>groupByLocationDay = df.groupby([df.id,
                                pd.Grouper(key='opened_at', freq='3h')])
</code></pre>

<p>I get the following</p>

<pre><code>id    opened_at             count   sum
      2016-07-01 06:00:00   2       4296.14
154   2016-07-01 09:00:00   46      43716.79
      2016-07-01 12:00:00   169     150827.14
      2016-07-02 12:00:00   17      1508.14
      2016-07-02 09:00:00   10      108.14
</code></pre>

<p>How can I group by hour only, so that it would look like the following.</p>

<pre><code>id    opened_at         count   sum
      06:00:00  2       4296.14
154   09:00:00  56      43824.93
      12:00:00  203     152335.28
</code></pre>

<p>The original data is on <strong>hourly basis</strong>, thus I need to get <strong>3h frequency</strong>.
Thanks!</p>