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explaining #set directive

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@@ -3751,6 +3751,50 @@ ephem.Eros = eros</pre>
a companion document <a href="sle.html"><em>Writing search list extensions</em></a>.
</p>
<h2 id="calculation_in_templates">How to calculate values in templates</h2>
<p>
While it is the best way to define calculated values by writing a search list extension or XTYPE extension,
sometimes it is easier or faster to include the calculation in the template. To do so, Cheetah provides
the <span class="code">#set</code> directive. Here it is explained how to use it for calculations and
displaying the resulting values.
</p>
<p>
To get an observation type value in a way that can be used in formulae you can add <span class="code">.raw</span>
after an observation type tag.
</p>
<p>
Example:
</p>
<pre class="tty">$almanac.sun.rise.raw</pre>
<p>
To calculate - for example - the daylight time:
</p>
<pre>#set $daylight=$almanac.sun.set.raw-$almanac.sun.rise.raw</pre>
<p>
To display that value and make it observe the units and settings is a little bit tricky:
</p>
<pre>#from weewx.units import ValueTuple,ValueHelper
#set $rising=$almanac.sun.rise.raw
#set $setting=$almanac.sun.set.raw
#set $daylight=$setting-$rising
#set $daylight_vh=ValueHelper(ValueTuple($daylight,'second','group_deltatime'),formatter=$station.formatter)
$daylight_vh</pre>
<p>The unit provided has to conform to the unit of the raw values used in calculation. </p>
<p>To get a special output format you can add <span class="code">.format(...)</code>, for example:</p>
<pre>$daylight_vh.format("%(hour)d%(hour_label) %(minute)d%(minute_label)s %(second)d%(second_label)s","")</pre>
<p>
Another example is the difference between maximum and minimum temperature of the day in Kelvin
(which is the same value as in degrees Centigrade for differences, so no unit conversion applies):
</p>
<pre>#from weewx.units import ValueTuple,ValueHelper
#set $maxmin=$day.outTemp.max.degree_C.raw-$day.outTemp.min.degree_C.raw
#set $maxmin_vh=ValueHelper(ValueTuple($maxmin,'degree_C','group_temperature'),formatter=$station.formatter)
$maxmin_vh.format(add_label=False) Kelvin</pre>
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