weewx/docs/xtypes.md

Extensible types

Abstract

This proposal allows new derived observation types to be added to WeeWX.

Motivation

Right now, the set of observation types is fixed:

• The set of types that StdWXCalculate knows how to calculate is limited to a set of "magic types," such as dewpoint, or heatindex.
• Heating and cooling degree days are also special types, defined in subclass WXDaySummaryManager.
• There is no way to add new types to the tag system, unless they appear in the "current" record, or in the database.

In theory, new types can be introduced by subclassing, but this allows only new types to be accreted in a linear fashion: it would not be possible for two extensions to both introduce new types. One would have to inherit from the other.

The goal of XTypes is to allow the user to add new types to the system with a minimum of fuss.

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Proposal

Summary

• A new module, weewx.xtypes, will be introduced. Similar to the existing weewx.units, it would be responsible for managing new types. New types can be added dynamically, using a Python API.

• The service StdWXCalculate will no longer have a fixed set of "special" types. Instead, it will be extensible, by using weewx.xtypes. The existing options of hardware, prefer_hardware, and software would continue. It would come out-of-the-box with the existing types it now handles (dewpoint, heatindex, etc.), but new types could be added by the user. This allows new types to appear in the current LOOP packet or archive record, allowing their use elsewhere in WeeWX.

• When resolving tags, the Cheetah generator would first look in the present record, then in the database, as it does now. But, then it would look to weewx.types to try and calculate any unresolved types. This would allow the products of StdWXCalculate to be used by wee_reports, resolving Issue #95

• In a similar manner, the Image generator would first try the database to resolve any series. If that doesn't work, it would then try weewx.xtypes.

• The class WXDaySummaryManager would be deprecated, and the two types heatdeg and cooldeg would no longer depend on it. Instead, the tag system would use weewx.xtypes to calculate them.

• The schema system would be expanded to allow explicit declaration of the schema for the daily summaries. This replaces some functionality presently done by WXDaySummaryManager.

Overview of adding new types

Adding a new observation type is done by subclassing the abstract base class XTypes, then overriding one to three functions:

class XTypes:
    get_scalar(obs_type, record, db_manager=None)
    get_series(obs_type, timespan, db_manager, aggregate_type=None, aggregate_interval=None)
    get_aggregate(obs_type, timespan, aggregate_type, db_manager, **option_dict)

An instance of the subclass is then instantiated and registered with weewx.xtypes. Note that it is not always necessary to supply all three functions. Details follow

Calculating scalars

To calculate a custom scalar value, the user should subclass XTypes, then override the member function get_scalar:

class MyTypes(XTypes):

    def get_scalar(obs_type, record, db_manager):
        ...

Where

• obs_type is the type to be computed.
• record is a WeeWX record. It will include at least types dateTime and usUnits.
• db_manager is an instance of weewx.manager.Manager, or a subclass. The connection will be open and usable.

The function should return:

• A ValueTuple scalar. The value held by the ValueTuple can be None.

The function should raise:

• An exception of type weewx.UnknownType, if the type obs_type is not known to the function.
• An exception of type weewx.CannotCalculate if the type is known to the function, but all the information necessary to calculate the type is not there.

Calculating series

The user should subclass XTypes, then override the member function get_series:

class MyTypes(XTypes):

    def get_series(obs_type, timespan, db_manager, aggregate_type, aggregate_interval):
       ...

Where

• obs_type is the type to be computed.
• timespan is an instance of weeutil.weeutil.TimeSpan. It defines bounding start and ending times of the series, exclusive on the left, inclusive on the right.
• db_manager is an instance of weewx.manager.Manager, or a subclass. The connection will be open and usable.
• aggregate_type defines the type of aggregation, if any. Typically, it is one of sum, avg, min, or max, although there is nothing stopping the user-defined extension from defining new types of aggregation. If set to None, then no aggregation should occur, and the full series should be returned.
• aggregate_interval is an aggregation interval. If aggregation is to be done (i.e., aggregate_type is not None), then the series should be grouped by the aggregation interval.

The function should return:

• A three-way tuple: (start_list_vt, stop_list_vt, data_list_vt) where

  • start_list_vt is a ValueTuple, whose first element is a list of start times;
  • stop_list_vt is a ValueTuple, whose first element is a list of stop times;
  • data_list_vt is a ValueTuple, whose first element is a list of aggregated values.

The function should raise:

• An exception of type weewx.UnknownType, if the type obs_type is not known to the function.
• An exception of type weewx.UnknownAggregation if the aggregation aggregate_type is not known to the function.
• An exception of type weewx.CannotCalculate if the type and aggregation are known to the function, but all the information necessary to perform the calculate is not there.

Calculating aggregates

To calculate a custom aggregation, the user should override the member function get_aggregate:

class MyTypes(XTypes):

    def get_aggregate(obs_type, timespan, aggregate_type, db_manager, **option_dict):
        ...

Where

• obs_type is the type over which the aggregation is to be computed.
• timespan is an instance of weeutil.weeutil.TimeSpan. It defines bounding start and ending times of the aggregation, exclusive on the left, inclusive on the right.
• aggregate_type is the type of aggregation to be performed, such as avg, or last, or it can be some new, user-defined aggregation.
• db_manager is an instance of weewx.manager.Manager, or a subclass. The connection will be open and usable.
• option_dict is a dictionary with possible, additional, values to be used by the aggregation. (Need details)

The function should return:

• A ValueTuple holding the aggregated value.

The function should raise:

• An exception of type weewx.UnknownType, if the type obs_type is not known to the function.
• An exception of type weewx.UnknownAggregation if the aggregation aggregate_type is not known to the function.
• An exception of type weewx.CannotCalculate if the type and aggregation are known to the function, but all the information necessary to perform the aggregation is not there.

Registering your subclass

The module weewx.xtypes keeps a simple list of extensions. Your new class should be prepended or appended to the list, depending on whether you want it to override other extensions.

import weewx.xtypes

class MyXType(weewx.xtypes.XType):
    def get_scalar(self, obs_type, record, db_manager=None):
        # Perform some calculation...
        return value

# Instantiate an instance, and append it to the list:
weewx.xtypes.xtypes.append(MyXType())

Using the extension

Module weewx.xtypes supplies 3 functions for using user-supplied extensions:

get_scalar(obs_type, record, db_manager=None)
get_series(obs_type, timespan, db_manager, aggregate_type=None, aggregate_interval=None)
get_aggregate(obs_type, timespan, aggregate_type, db_manager, **option_dict)

Example: function weewx.xtypes.get_scalar() searches the list weewx.xtypes.xtypes, trying member function get_scalar() of each object in turn. If the member function raises weewx.UnknownType or weewx.CannotCalculate, weewx.xtypes.get_scalar() moves on to the next object in the list. If no function can be found to do the evaluation, it raises weewx.Unknowntype.

The other functions work in a similar manner.

Example

File user/pressure.py

import weewx.units
import weewx.uwxutils
import weewx.xtypes

class Pressure(weewx.xtypes.Xtype):

    def __init__(self, altitude_ft):
        """Initialize  with the altitude in feet"""
        self.altitude_ft = altitude_ft

    def _get_temperature_12h(self, ts, dbmanager):
        """Get the temperature from 12 hours ago."""

        # ... details elided

    def get_scalar(self, obs_type, record, dbmanager):
        """Calculate the observation type 'pressure'."""

        if obs_type != 'pressure':
            raise weewx.UnknownType

        # Get the temperature in Fahrenheit from 12 hours ago
        temp_12h_F = self._get_temperature_12h(record['dateTime'], dbmanager)
        if temp_12h_F is not None:
            try:
                # The following requires everything to be in US Customary units.
                record_US = weewx.units.to_US(record)
                pressure = weewx.uwxutils.uWxUtilsVP.SeaLevelToSensorPressure_12(
                    record_US['barometer'],
                    self.altitude_ft,
                    record_US['outTemp'],
                    temp_12h_F,
                    record_US['outHumidity']
                )

                if record['usUnits'] == weewx.METRIC or record['usUnits'] == weewx.METRICWX:
                    pressure /= weewx.units.INHG_PER_MBAR
                return pressure

            except KeyError:
                # Don't have everything we need. Raise an exception.
                raise weewx.CannotCalculate(obs_type)

        # Else, fall off the end and return None

Note how the method get_scalar() raises an exception of type weewx.UnknownType for any types it does not recognize, that is, any type other than pressure.

Also, note that the method requires observation types barometer, outTemp, and outHumidity in order to perform the calculation, and raises an exception of type weewx.CannotCalculate if one of them is missing.

Registering the extension

Continuing our example above:

import weewx.xtypes

# Create an instance of the Pressure class, initializing it with the altitude in feet:
pobj = Pressure(700.0)
# Register the the instance:
weewx.xtypes.xtypes.append(pobj)

Using the extension

To use the above example:

import weewx.manager

archive_sqlite = {'database_name': '/home/weewx/archive/weewx.sdb', 'driver': 'weedb.sqlite'}

with weewx.manager.Manager.open_with_create(archive_sqlite) as db_manager:
    # Work with the last record in the database:
    timestamp = db_manager.lastGoodStamp()
    record = db_manager.getRecord(timestamp)
    p = weewx.xtypes.get_scalar('pressure', record, db_manager)
    print("Pressure = %s" % p)

    # Try again, but missing outTemp:
    del record['outTemp']
    try:
        p = weewx.xtypes.get_scalar('pressure', record, db_manager)
    except weewx.CannotCalculate as e:
        print("Unable to calculate type '%s'" % e)

    # Try calculating a type we know nothing about
    try:
        q = weewx.xtypes.get_scalar('foo', record, db_manager)
    except weewx.UnknownType as e:
        print("Unknown type: '%s'" % e)

Results of running the program:

Pressure = 29.4539701889
Unable to calculate type `pressure`
Unknown type: 'foo'

The function weewx.xtypes.get_scalar() will try each registered object in order. For each object, it calls member function get_scalar(). If an exception of type weewx.UnknownType is raised, it moves on to the next one, continuing until it receives a value. If no registered instance knows how to perform the calculation, then weewx.xtypes.get_scalar() itself will raise an exception of type weewx.UnknownType. Callers should be prepared to catch it, depending on context.

A more comprehensive example

See the repository weepwr, for a more complex example. This is a device driver for the Brultech energy monitors. It registers many new types, and does this dynamically.

Alternatives to the chosen design

Alternative: register functions with weewx.conf

The chosen design registered new types through a Python API. An alternative is to declare the types and function to be called in weewx.conf, in a manner similar to search list extensions. This approach has the advantage that it requires a bit less programming and, most importantly, it leaves a concise record of what extensions are being used in the configuration file weewx.conf.

However, it has a big disadvantage. The example above shows why. It is difficult to predict what data a user might need to write an extension. In our example, we needed the altitude of the station. Where would that come from? The answer is that it would have to be supplied by a standardized interface to the user function, which would make all manner of information available. This means the user might potentially have to know everything, so you end up with a system where everything is connected to everything.

This is avoided by supplying a Python API that the type must adhere to. The new type can get any information it wants, then register with the API. This is what our example does.

Alternative: register functions through the API

With this alternative, new types register with a Python API, but register functions, rather than instances of classes.

The disadvantage is that this results in a proliferation of small functions. The chosen method has the advantage that all the functions needed for a type can be held held under one roof.

Alternative: declare types

In the chosen design, if an observation type is not known to a type extension, it raises an exception of type weewx.UnknownType. An alternative that was considered is to require extensions to declare what types they can handle in advance, much like a dictionary. This allows types to be discoverable.

But, it has a disadvantage that all known types must be declared. That's not always practical. For example, a type extension would be useful to calculate power from a running aggregate of energy from an energy monitor. But, some monitors are capable of emitting 48+ channels, each of which would have to be declared. Add polarized and absolute values, and we are looking at nearly 100 types!

Instead, we allow extensions to recognize what types they know about, possibly from a regular expression of the observation type. If they don't recognize the type, they raise weewx.UnknownType. Types do not have to be declared in advance.

Open issues