Skip to content

Writing search list extensions

The intention of this document is to help you write new Search List Extensions (SLE). Here's the plan:

  • We start by explaining how SLEs work.

  • Then we will look at an example that implements an extension $seven_day().

  • Then we will look at another example, $colorize(), which allows you to pick background colors on the basis of a value (for example, low temperatures could show blue, while high temperatures show red). It will be implemented using 3 different, increasingly sophisticated, ways:

    • A simple, hardwired version that works in only one unit system.

    • A version that can handle any unit system, but with the colors still hardwared.

    • Finally, a version that can handle any unit system, and takes its color bands from the configuration file.

How the search list works

Let's start by taking a look at how the Cheetah search list works.

The Cheetah template engine finds tags by scanning a search list, a Python list of objects. For example, for a tag $foo, the engine will scan down the list, trying each object in the list in turn. For each object, it will first try using foo as an attribute, that is, it will try evaluating If that raises an AttributeError exception, then it will try foo as a key, that is obj[key]. If that raises a KeyError exception, then it moves on to the next item in the list. The first match that does not raise an exception is used. If no match is found, Cheetah raises a NameMapper.NotFound exception.

A simple tag

Now let's take a look at how the search list interacts with WeeWX tags. Let's start by looking at a simple example: station altitude, available as the tag


As we saw in the previous section, Cheetah will run down the search list, looking for an object with a key or attribute station. In the default search list, WeeWX includes one such object, an instance of the class weewx.cheetahgenerator.Station, which has an attribute station, so it gets a hit on this object.

Cheetah will then try to evaluate the attribute altitude on this object. Class Station has such an attribute, so Cheetah evaluates it.

Return value

What this attribute returns is not a raw value, say 700, nor even a string. Instead, it returns an instance of the class ValueHelper, a special class defined in module weewx.units. Internally, it holds not only the raw value, but also references to the formats, labels, and conversion targets you specified in your configuration file. Its job is to make sure that the final output reflects these preferences. Cheetah doesn't know anything about this class. What it needs, when it has finished evaluating the expression $station.altitude, is a string. In order to convert the ValueHelper it has in hand into a string, it does what every other Python object does when faced with this problem: it calls the special method __str__. Class ValueHelper has a definition for this method. Evaluating this function triggers the final steps in this process. Any necessary unit conversions are done, then formatting occurs and, finally, a label is attached. The result is a string something like

700 feet

which is what Cheetah actually puts in the generated HTML file. This is a good example of lazy evaluation. The tags gather all the information they need, but don't do the final evaluation until the last final moment, when the most context is understood. WeeWX uses this technique extensively.

A slightly more complex tag

Now let's look at a more complicated example, say the maximum temperature since midnight:


When this is evaluated by Cheetah, it actually produces a chain of objects. At the top of this chain is class weewx.tags.TimeBinder, an instance of which is included in the default search list. Internally, this instance stores the time of the desired report (usually the time of the last archive record), a cache to the databases, a default data binding, as well as references to the formatting and labelling options you have chosen.

This instance is examined by Cheetah to see if it has an attribute day. It does and, when it is evaluated, it returns the next class in the chain, an instance of weewx.tags.TimespanBinder. In addition to all the other things contained in its parent TimeBinder, class TimespanBinder adds the desired time period, that is, the time span from midnight to the current time.

Cheetah then continues on down the chain and tries to find the next attribute, outTemp. There is no such hard coded attribute (hard coding all the conceivable different observation types would be impossible!). Instead, class TimespanBinder defines the Python special method __getattr__. If Python cannot find a hard coded version of an attribute, and the method __getattr__ exists, it will try it. The definition provided by TimespanBinder returns an instance of the next class in the chain, weewx.tags.ObservationBinder, which not only remembers all the previous stuff, but also adds the observation type, outTemp.

Cheetah then tries to evaluate an attribute max of this class, and the pattern repeats. Class weewx.tags.ObservationBinder does not have an attribute max, but it does have a method __getattr__. This method returns an instance of the next class in the chain, class AggTypeBinder, which not only remembers all the previous information, but adds the aggregation type, max.

One final step needs to occur: Cheetah has an instance of AggTypeBinder in hand, but what it really needs is a string to put in the file being created from the template. It creates the string by calling the method __str__() of AggTypeBinder. Now, finally, the chain ends and everything comes together. The method __str__ triggers the actual calculation of the value, using all the known parameters: the database binding to be hit, the time span of interest, the observation type, and the type of aggregation, querying the database as necessary. The database is not actually hit until the last possible moment, after everything needed to do the evalation is known.

Like our previous example, the results of the evaluation are then packaged up in an instance of ValueHelper, which does the final conversion to the desired units, formats the string, then adds a label. The results, something like


are put in the generated HTML file. As you can see, a lot of machinery is hidden behind the deceptively simple expression $day.outTemp.max!

Extending the list

As mentioned, WeeWX comes with a number of objects already in the search list, but you can extend it.

The general pattern is to create a new class that inherits from weewx.cheetahgenerator.SearchList, which supplies the functionality you need. You may or may not need to override its member function get_extension_list(). If you do not, then a default is supplied.

Adding tag $seven_day

Let's look at an example. The regular version of WeeWX offers statistical summaries by day, week, month, year, rain year, and all time. While WeeWX offers the tag $week, this is statistics since Sunday at midnight. Suppose we would like to have statistics for a full week, that is since midnight seven days ago.

If you wish to use or modify this example, cut and paste the below to user/

import datetime
import time

from weewx.cheetahgenerator import SearchList
from weewx.tags import TimespanBinder
from weeutil.weeutil import TimeSpan

class SevenDay(SearchList):                                                  # 1

    def __init__(self, generator):                                           # 2
        SearchList.__init__(self, generator)

    def get_extension_list(self, timespan, db_lookup):                       # 3
        """Returns a search list extension with two additions.

          timespan: An instance of weeutil.weeutil.TimeSpan. This will
                    hold the start and stop times of the domain of
                    valid times.

          db_lookup: This is a function that, given a data binding
                     as its only parameter, will return a database manager

        # Create a TimespanBinder object for the last seven days. First,
        # calculate the time at midnight, seven days ago. The variable week_dt 
        # will be an instance of
        week_dt = \
                  - datetime.timedelta(weeks=1)                              # 4
        # Convert it to unix epoch time:
        week_ts = time.mktime(week_dt.timetuple())                           # 5
        # Form a TimespanBinder object, using the time span we just
        # calculated:
        seven_day_stats = TimespanBinder(TimeSpan(week_ts, timespan.stop),
                                         skin_dict=self.generator.skin_dict) # 6

        # Now create a small dictionary with the key 'seven_day':
        search_list_extension = {'seven_day' : seven_day_stats}              # 7

        # Finally, return our extension as a list:
        return [search_list_extension]                                       # 8

Going through the example, line by line:

  1. Create a new class called SevenDay, which will inherit from class SearchList. All search list extensions must inherit from this class.
  2. Create an initializer for our new class. In this case, the initializer is not really necessary and does nothing except pass its only parameter, generator, a reference to the calling generator, on to its superclass, SearchList, which will then store it in self. Nevertheless, we include the initializer in case you wish to modify it.
  3. Override member function get_extension_list(). This function will be called when the generator is ready to accept your new search list extension. The parameters that will be passed in are:
    • self Python's way of indicating the instance we are working with;
    • timespan An instance of the utility class TimeSpan. This will contain the valid start and ending times used by the template. Normally, this is all valid times, but if your template appears under one of the "Summary By" sections in the [CheetahGenerator] section of skin.conf, then it will contain the timespan of that time period.
    • db_lookup This is a function supplied by the generator. It takes a single argument, a name of a binding. When called, it will return an instance of the database manager class for that binding. The default for the function is whatever binding you set with the option data_binding for this report, usually wx_binding.
  4. The object timespan holds the domain of all valid times for the template, but in order to calculate statistics for the last seven days, we need not the earliest valid time, but the time at midnight seven days ago. So, we do a little Python date arithmetic to calculate this. The object week_dt will be an instance of
  5. We convert it to unix epoch time and assign it to variable week_ts.
  6. The class TimespanBinder represents a statistical calculation over a time period. We have already met it in the introduction. In our case, we will set it up to represent the statistics over the last seven days. The class takes 6 parameters.
    • The first is the timespan over which the calculation is to be done, which, in our case, is the last seven days. In step 5, we calculated the start of the seven days. The end is "now", that is, the end of the reporting period. This is given by the end point of timespan, timespan.stop.
    • The second, db_lookup, is the database lookup function to be used. We simply pass in db_lookup.
    • The third, context, is the time context to be used when formatting times. The set of possible choices is given by sub-section [[TimeFormats]] in the configuration file. Our new tag, $seven_day is pretty similar to $week, so we will just use 'week', indicating that we want a time format that is suitable for a week-long period.
    • The fourth, formatter, should be an instance of class weewx.units.Formatter, which contains information about how the results should be formatted. We just pass in the formatter set up by the generator, self.generator.formatter.
    • The fifth, converter, should be an instance of weewx.units.Converter, which contains information about the target units (e.g., degree_C) that are to be used. Again, we just pass in the instance set up by the generator, self.generator.converter.
    • The sixth, skin_dict, is an instance of configobj.ConfigObj, and contains the contents of the skin configuration file. We pass it on in order to allow aggregations that need information from the file, such as heating and cooling degree-days.
  7. Create a small dictionary with a single key, seven_day, whose value will be the TimespanBinder that we just constructed.
  8. Return the dictionary in a list


The final step that we need to do is to tell the template engine where to find our extension. You do that by going into the skin configuration file, skin.conf, and adding the option search_list_extensions with our new extension. When you're done, it will look something like this:

    # This section is used by the generator CheetahGenerator, and specifies
    # which files are to be generated from which template.

    # Possible encodings include 'html_entities', 'strict_ascii', 'normalized_ascii',
    # as well as those listed in
    encoding = html_entities
    search_list_extensions = user.seven_day.SevenDay


Our addition has been highlighted. Note that it is in the section [CheetahGenerator].

Now, if the Cheetah engine encounters the tag $seven_day, it will scan the search list, looking for an attribute or key that matches seven_day. When it gets to the little dictionary we provided, it will find a matching key, allowing it to retrieve the appropriate TimespanBinder object.

With this approach, you can now include "seven day" statistics in your HTML templates:

        <td>Maximum temperature over the last seven days:</td>
        <td>Minimum temperature over the last seven days:</td>
        <td>Rain over the last seven days:</td>

We put our addition in the "user" directory, which is automatically included by WeeWX in the Python path. However, if you put the file somewhere else, you may have to specify its location with the environment variable PYTHONPATH when you start WeeWX:

export PYTHONPATH=/home/me/secret_location

Adding tag $colorize

Let's look at another example. This one will allow you to supply a background color, depending on the temperature. For example, to colorize an HTML table cell:



    <td>Outside temperature</td>
    <td style="background-color:$colorize($current.outTemp.raw)">$current.outTemp</td>



The highlighted expression will return a color, depending on the value of its argument. For example, if the temperature was 30.9ºF, then the output might look like:

Outside temperature 30.9°F

A very simple implementation

We will start with a very simple version. The code can be found in examples/colorize/

from weewx.cheetahgenerator import SearchList

class Colorize(SearchList):                                          # 1

    def colorize(self, t_c):                                         # 2
        """Choose a color on the basis of temperature

            t_c (float): The temperature in degrees Celsius

            str: A color string

        if t_c is None:                                              # 3
            return "#00000000"
        elif t_c < -10:
            return "magenta"
        elif t_c < 0:
            return "violet"
        elif t_c < 10:
            return "lavender"
        elif t_c < 20:
            return "mocassin"
        elif t_c < 30:
            return "yellow"
        elif t_c < 40:
            return "coral"
            return "tomato"

The first thing that's striking about this version is just how simple an SLE can be: just one class with a single function. Let's go through the implementation line-by-line.

  1. Just like the first example, all search list extensions inherit from weewx.cheetahgenerator.SearchList

  2. The class defines a single function, colorize(), with a single argument that must be of type float.

    Unlike the first example, notice how we do not define an initializer, __init__(), and, instead, rely on our superclass to do the initialization.

  3. The function relies on a big if/else statement to pick a color on the basis of the temperature value. Note how it starts by checking whether the value could be Python None. WeeWX uses None to represent missing or invalid data. One must be always vigilant in guarding against a None value. If None is found, then the color #00000000 is returned, which is transparent and will have no effect.


As before, we must register our extension with the Cheetah engine. We do this by copying the extension to the user directory, then adding its location to option search_list_extensions:

    search_list_extensions = user.colorize_1.Colorize

Where is get_extension_list()?

You might wonder, "What happened to the member function get_extension_list()? We needed it in the first example; why not now?" The answer is that we are inheriting from, and relying on, the version in the superclass SearchList, which looks like this:

    def get_extension_list(self, timespan, db_lookup):
        return [self]

This returns a list, with itself (an instance of class Colorize) as the only member.

How do we know whether to include an instance of get_extension_list()? Why did we include a version in the first example, but not in the second?

The answer is that many extensions, including $seven_day, need information that can only be known when the template is being evaluated. In the case of $seven_day, this was which database binding to use, which will determine the results of the database query done in its implementation. This information is not known until get_extension_list() is called, which is just before template evaluation.

By constrast, $colorize() is pure static: it doesn't use the database at all, and everything it needs it can get from its single function argument. So, it has no need for the information in get_extension_list().


Let's review the whole process. When the WeeWX Cheetah generator starts up to evaluate a template, it first creates a search list. It does this by calling get_extension_list() for each SLE that has been registered with it. In our case, this will cause the function above to put an instance of Colorize in the search list — we don't have to do anything to make this happen.

When the engine starts to process the template, it will eventually come to

<td style="background-color:$colorize($current.outTemp.raw)">$current.outTemp</td>

It needs to evaluate the expression $colorize($current.outTemp.raw), so it starts scanning the search list looking for something with an attribute or key colorize. When it comes to our instance of Colorize it gets a hit because, in Python, member functions are implemented as attributes. The Cheetah engine knows to call it as a function because of the parenthesis that follow the name. The engine passes in the value of $current.outTemp.raw as the sole argument, where it appears under the name t_c.

As described above, the function colorize() then uses the argument to choose an appropriate color, returning it as a string.


This example has an obvious limitation: the argument to $colorize() must be in degrees Celsius. We can guard against passing in the wrong unit by always converting to Celsius first:

<td style="background-color:$colorize($current.outTemp.degree_C.raw)">$current.outTemp</td>

but the user would have to remember to do this every time colorize() is called. The next version gets around this limitation.

A slightly better version

Here's an improved version that can handle an argument that uses any unit, not just degrees Celsius. The code can be found in examples/colorize/

import weewx.units
from weewx.cheetahgenerator import SearchList

class Colorize(SearchList):                                          # 1

    def colorize(self, value_vh):                                    # 2
        """Choose a color string on the basis of a temperature value"""

        # Extract the ValueTuple part out of the ValueHelper
        value_vt = value_vh.value_t                                  # 3

        # Convert to Celsius:
        t_celsius = weewx.units.convert(value_vt, 'degree_C')        # 4

        # The variable "t_celsius" is a ValueTuple. Get just the value:
        t_c = t_celsius.value                                        # 5

        # Pick a color based on the temperature
        if t_c is None:                                              # 6
            return "#00000000"
        elif t_c < -10:
            return "magenta"
        elif t_c < 0:
            return "violet"
        elif t_c < 10:
            return "lavender"
        elif t_c < 20:
            return "mocassin"
        elif t_c < 30:
            return "yellow"
        elif t_c < 40:
            return "coral"
            return "tomato"

Going through the example, line by line:

  1. Just like the other examples, we must inherit from weewx.cheetahgenerator.SearchList.

  2. However, in this example, notice that the argument to colorize() is an instance of class ValueHelper, instead of a simple float.

    As before, we do not define an initializer, __init__(), and, instead, rely on our superclass to do the initialization.

  3. The argument value_vh will contain many things, including formatting and preferred units, but, for now, we are only interested in the ValueTuple contained within, which can be extracted with the attribute value_t.

  4. The variable value_vt could be in any unit that measures temperature. Our code needs Celsius, so we convert to Celsius using the convenience function weewx.units.convert(). The results will be a new ValueTuple, this time in Celsius.

  5. We need just the temperature value, and not the other things in a ValueTuple, so extract it using the attribute value. The results will be a simple instance of float or, possibly, Python None.

  6. Finally, we need a big if/else statement to choose which color to return, while making sure to test for None.

This version uses a ValueHelper as an argument instead of a float. How do we call it? Here's an example:



    <td>Outside temperature</td>
    <td style="background-color:$colorize($current.outTemp)">$current.outTemp</td>



This time, we call the function with a simple $current.outTemp (without the .raw suffix), which is actually an instance of class ValueHelper. When we met this class earlier, the Cheetah engine needed a string to put in the template, so it called the special member function __str__(). However, in this case, the results are going to be used as an argument to a function, not as a string, so the engine simply passes in the ValueHelper unchanged to colorize(), where it appears as argument value_vh.

Our new version is better than the original because it can take a temperature in any unit, not just Celsius. However, it can still only handle temperature values and, even then, the color bands are still hardwired in. Our next version will remove these limitations.

A more sophisticated version

Rather than hardwire in the values and observation type, in this version we will retrieve them from the skin configuration file, skin.conf. Here's what a typical configuration might look like for this version:

[Colorize]                      # 1
    [[group_temperature]]       # 2
        unit_system = metricwx  # 3
        default = tomato        # 4
        None = lightgray        # 5
        [[[upper_bounds]]]      # 6
            -10 = magenta       # 7
            0 = violet          # 8
            10 = lavender
            20 = mocassin
            30 = yellow
            40 = coral
    [[group_uv]]                # 9
        unit_system = metricwx
        default = darkviolet
            2.4 = limegreen
            5.4 = yellow
            7.4 = orange
            10.4 = red

Here's what the various lines in the configuration stanza mean:

  1. All the configuration information needed by the SLE Colorize can be found in a stanza with the heading [Colorize]. Linking facility with a stanza of the same name is a very common pattern in WeeWX.
  2. We need a separate color table for each unit group that we are going to support. This is the start of the table for unit group group_temperature.
  3. We need to specify what unit system will be used by the temperature color table. In this example, we are using metricwx.
  4. In case we do not find a value in the table, we need a default. We will use the color tomato.
  5. In case the value is Python None, return the color given by option None. We will use lightgray.
  6. The sub-subsecction [[[upper_bounds]]] lists the upper (max) value of each of the color bands.
  7. The first color band (magenta) is used for temperatures less than or equal to -10°C.
  8. The second band (violet) is for temperatures greater than -10°C and less than or equal to 0°C. And so on.
  9. The next subsection, [[group_uv]], is very similar to the one for group_temperature, except the values are for bands of the UV index.

Although [Colorize] is in skin.conf, there is nothing special about it, and it can be overridden in weewx.conf, just like any other configuration information.

Annotated code

Here's the alternative version of colorize(), which will use the values in the configuration file. It can also be found in examples/colorize/

import weewx.units
from weewx.cheetahgenerator import SearchList

class Colorize(SearchList):                                               # 1

    def __init__(self, generator):                                        # 2
        SearchList.__init__(self, generator)
        self.color_tables = self.generator.skin_dict.get('Colorize', {})

    def colorize(self, value_vh):

        # Get the ValueTuple and unit group from the incoming ValueHelper
        value_vt = value_vh.value_t                                       # 3
        unit_group =                                       # 4

        # Make sure unit_group is in the color table, and that the table
        # specifies a unit system.
        if unit_group not in self.color_tables \
                or 'unit_system' not in self.color_tables[unit_group]:    # 5
            return "#00000000"

        # Convert the value to the same unit used by the color table:
        unit_system = self.color_tables[unit_group]['unit_system']        # 6
        converted_vt = weewx.units.convertStdName(value_vt, unit_system)  # 7

        # Check for a value of None
        if converted_vt.value is None:                                    # 8
            return self.color_tables[unit_group].get('none') \
                   or self.color_tables[unit_group].get('None', "#00000000")

        # Search for the value in the color table:
        for upper_bound in self.color_tables[unit_group]['upper_bounds']: # 9
            if converted_vt.value <= float(upper_bound):                  # 10
                return self.color_tables[unit_group]['upper_bounds'][upper_bound]

        return self.color_tables[unit_group].get('default', "#00000000")  # 11
  1. As before, our class must inherit from SearchList.

  2. In this version, we supply an initializer because we are going to do some work in it: extract our color table out of the skin configuration dictionary. In case the user neglects to include a [Colorize] section, we substitute an empty dictionary.

  3. As before, we extract the ValueTuple part out of the incoming ValueHelper using the attribute value_t.

  4. Retrieve the unit group used by the incoming argument. This will be something like "group_temperature".

  5. What if the user is requesting a color for a unit group that we don't know anything about? We must check that the unit group is in our color table. We must also check that a unit system has been supplied for the color table. If either of these checks fail, then return the color #00000000, which will have no effect in setting a background color.

  6. Thanks to the checks we did in step 5, we know that this line will not raise a KeyError exception. Get the unit system used by the color table for this unit group. It will be something like 'US', 'metric', or 'metricwx'.

  7. Convert the incoming value, so it uses the same units as the color table.

  8. We must always be vigilant for values of Python None! The expression

    self.color_tables[unit_group].get('none') or self.color_tables[unit_group].get('None', "#00000000")

    is just a trick to allow us to accept either "none" or "None" in the configuration file. If neither is present, then we return the color #00000000, which will have no effect.

  9. Now start searching the color table to find a band that is less than or equal to the value we have in hand.

  10. Two details to note.

    First, the variable converted_vt is a ValueTuple. We need the raw value in order to do the comparison. We get this through attribute .value.

    Second, WeeWX uses the utility ConfigObj to read configuration files. When ConfigObj returns its results, the values will be strings. We must convert these to floats before doing the comparison. You must be constantly vigilant about this when working with configuration information.

    If we find a band with an upper bound greater than our value, we have a hit. Return the corresponding color.

  11. If we make it all the way through the table without a hit, then we must have a value greater than anything in the table. Return the default, or the color #00000000 if there is no default.