Extensions¶

A key feature of WeeWX is its ability to be extended by installing 3^rd party extensions. Extensions are a way to package one or more customizations so that they can be installed and distributed as a functional group.

Customizations typically fall into one of these categories:

• driver
• skin
• search list extension
• service
• generator

Take a look at the WeeWX wiki for a sampling of some of the extensions that are available.

Guidelines¶

Now that you have made some customizations, you might want to share those changes with other WeeWX users. Put your customizations into an extension to make installation, removal, and distribution easier.

Here are a few guidelines for creating extensions:

• Extensions should not modify or depend on existing skins. An extension should include its own, standalone skin to illustrate any templates, search list extension, or generator features.

• Extensions should not modify the database schemas. If it requires data not found in the default databases, an extension should provide its own database and schema.

• Extensions that require some measure of configuration for them to work should not be enabled by default. Otherwise, they will crash the system on startup.

• Extensions should generally have their own stanza in weewx.conf. Be sure to list all possible options in it, albeit commented out. This way the user will know what is available.

• Although one extension might use another extension, take care to write the dependent extension so that it fails gracefully. For example, a skin might use data from the forecast extension, but what happens if the forecast extension is not installed? Make the skin display a message about "forecast not installed" but otherwise continue to function.

Packaging an extension¶

The structure of an extension mirrors that of WeeWX itself. If the customizations include a skin, the extension will have a skins directory. If the customizations include python code, the extension will have a bin/user directory.

Each extension should also include:

• readme.txt or readme.md - a summary of what the extension does, a list of pre-requisites (if any), and instructions for installing the extension manually

• changelog - an enumeration of changes in each release

• install.py - python code used by the WeeWX ExtensionInstaller. More details below.

For example, here is the structure of an extension called basic, which installs a skin called Basic. You can find it in the examples subdirectory.

basic
├── changelog
├── install.py
├── readme.md
└── skins
    └── Basic
        ├── basic.css
        ├── current.inc
        ├── favicon.ico
        ├── hilo.inc
        ├── index.html.tmpl
        ├── lang
        │   ├── en.conf
        │   └── fr.conf
        └── skin.conf

Here is the structure of an extension called xstats, which implements a search list extension, as well as a simple skin. You can also find it in the examples subdirectory.

xstats
├── bin
│   └── user
│       └── xstats.py
├── changelog
├── install.py
├── readme.txt
└── skins
    └── xstats
        ├── index.html.tmpl
        └── skin.conf

To distribute an extension, simply create a compressed archive of the extension directory.

For example, create the compressed archive for the basic skin like this:

tar cvfz basic.tar.gz basic

Once an extension has been packaged, it can be installed using weectl:

weectl extension install EXTENSION-LOCATION

Default values¶

Whenever possible, an extension should just work, with a minimum of input from the user. At the same time, parameters for the most frequently requested options should be easily accessible and easy to modify. For skins, this might mean parameterizing strings into [Labels] for easier customization. Or it might mean providing parameters in [Extras] to control skin behavior or to parameterize links.

Some parameters must be specified, and no default value would be appropriate. For example, an uploader may require a username and password, or a driver might require a serial number or IP address. In these cases, use a default value in the configuration that will obviously require modification. The username might default to REPLACE_ME. Also be sure to add a log entry that indicates the feature is disabled until the value has been specified.

Writing an installer¶

The installer is a python script called install.py that must be included in your extension. It is used to specify the various parts of the extension, where they should be put, and how they might be customized. To illustrate, let's take a look at the install.py script for the basic skin. Here it is:

def loader():                                                                # 1
    return BasicInstaller()


# By creating the configuration dictionary from a StringIO, we can preserve any comments
                                                                             # 2
BASIC_CONFIG = """
[StdReport]

    [[BasicReport]]
        skin = Basic
        enable = True
        # Language to use:
        lang = en
        # Unit system to use:
        unit_system = US
        # Where to put the results:
        HTML_ROOT = basic
"""

basic_dict = configobj.ConfigObj(StringIO(BASIC_CONFIG))                     # 3


class BasicInstaller(ExtensionInstaller):                                    # 4
    def __init__(self):
        super(BasicInstaller, self).__init__(                                # 5
            version="0.5",
            name='basic',
            description='Very basic skin for WeeWX.',
            author="Matthew Wall",
            author_email="mwall@users.sourceforge.net",
            config=basic_dict,                                               # 6
            files=[                                                          # 7
                ('skins/Basic',
                 ['skins/Basic/basic.css',
                  'skins/Basic/current.inc',
                  'skins/Basic/favicon.ico',
                  'skins/Basic/hilo.inc',
                  'skins/Basic/index.html.tmpl',
                  'skins/Basic/skin.conf',
                  'skins/Basic/lang/en.conf',
                  'skins/Basic/lang/fr.conf',
                  ]),
            ]
        )

    def configure(self, engine):                                             # 8
        """Customized configuration that sets a language code"""
        my_skin_path = os.path.join(os.path.dirname(__file__), 'skins/Basic')
        code = engine.get_lang_code(my_skin_path, 'en')
        self['config']['StdReport']['BasicReport']['lang'] = code
        return True

Going through this script line by line:

1. Every installer must define a loader() function that returns an instance of the installer class.

2. While you could specify the configuration dictionary as a Python structure, here we prefer to do it by creating and parsing a StringIO object. This has the advantage of preserving any comments in the configuration file.

3. Parse the configuration dictionary from the StringIO object.

4. Every installer must include an installer class that is a subclass of ExtensionInstaller. Here our class is called BasicInstaller.

5. The initializer for the installer class must call the initializer for the superclass, then initialize itself. This is where the various parts of the extension are specified.

6. On this line, we set the configuration dictionary for our extension. Whatever appears in this dictionary will override and augment the configuration file weewx.conf. In this example, in the [StdReport] stanza, a new [[BasicReport]] stanza will be created. If there is already a [[BasicReport]] stanza, perhaps because we are upgrading, it will be overwritten.

7. The files attribute of the installer class is a list of tuples that specify the files to be installed. The first element of the tuple is the destination directory, and the second element is a list of files to be installed in that directory. In this case, the destination directory is skins/Basic.

8. The configure() method is called by the extension installer to allow any custom configuration to be performed. In this case, we use it to ask the user which language s/he wants, then set a language code appropriately. If the custom configuration is successful, the function should return True, otherwise False.

Passing arguments on to your installer¶

It is possible to pass on additional command line arguments to your installer. To do this, declare a method process_args(self,args) in your installer class. Any arguments not recognized by weectl extension install will be passed on to it.

For example, let's modify the Basic skin example above so that if a --lang option is specified on the command line when installing the extension, the user is not asked which language to use.

import argparse

class BasicInstaller(ExtensionInstaller):
    def __init__(self):
        super().__init__(
            version="0.5",
            # ... as before ...

    def process_args(self, args):
        parser = argparse.ArgumentParser()
        parser.add_argument('--lang')
        namespace = parser.parse_args(args)
        # Set the language code or NONE, if no code was specified.
        self.lang = namespace.lang

    def configure(self, engine):
        """Customized configuration that sets a language code"""
        # If no language was specified on the command line, ask the user
        if not self.lang:
            my_skin_path = os.path.join(os.path.dirname(__file__), 'skins/Basic')
            self.lang = engine.get_lang_code(my_skin_path, 'en')
        self['config']['StdReport']['BasicReport']['lang'] = self.lang
        return True