Presets

In the case of Django (or other) you might want to use the presets system. By example, imagine that you’re always using the same set of settings and only a handful of values need to change from project to project. You can pack everything into one preset and let it modify what you need.

Preset interface

We’ll be working on a simple preset that will help by enabling I18N and setting the default language to the first language of the list.

We want to replace something like this:

LANGUAGE_CODE = "en"

LANGUAGES = [
    ("en", "English"),
    ("fr", "French"),
]

USE_I18N = True

By something like that:

from model_w.env_manager import EnvManager
from your_package.conf import I18nPreset

with EnvManager(I18nPreset()):
    LANGUAGES = [
        ("en", "English"),
        ("fr", "French"),
    ]

Here we suppose that your_package is some kind of internal package that you share between your projects.

Making the preset

Presets just need to implement the Preset interface. It simply has two hooks (pre and post) that allow you to set configuration values from the preset.

from model_w.env_manager import Preset
from typing import MutableMapping, Any

class I18nPreset(Preset):
    def pre(self, env: "EnvManager", context: MutableMapping[str, Any]):
        context['USE_I18N'] = True

    def post(self, env: "EnvManager", context: MutableMapping[str, Any]):
        context['LANGUAGE_CODE'] = context['LANGUAGES'][0][0]

Here in the pre hook we define static values while in the post hook we look at what the developer put in LANGUAGES to define what is going to be the default language (aka the first one of the list).

This way, you just need to set LANGUAGES and all I18N settings are set!

Composing the preset

Maybe you want also to have a preset to set the time zone. Let’s call it the TzPreset.

from model_w.env_manager import Preset
from typing import MutableMapping, Any

class TzPreset(Preset):
    def __init__(self, tz: str):
        self.tz = tz

    def post(self, env: "EnvManager", context: MutableMapping[str, Any]):
        context['USE_TZ'] = True
        context['TIME_ZONE'] = self.tz

Here we just set enable the time zone management and we make sure to set the time zone to a value specified by the user.

Now how do you compose this with the other one?

from model_w.env_manager import EnvManager, ComposePreset
from your_package.conf import I18nPreset, TzPreset

with EnvManager(ComposePreset(I18nPreset(), TzPreset('UTC'))):
    LANGUAGES = [
        ("en", "English"),
        ("fr", "French"),
    ]

Auto Preset

Another way to split concerns is to use the AutoPreset helper, which will automatically introspect your sub-class and call functions respecting a given convention.

For example, this demo preset:

from model_w.env_manager import AutoPreset

class FooFoo(AutoPreset):
    def pre_foo(self):
        yield 'FOO', 42

    def pre_bar(self):
        yield 'BAR', 24

    def post_foo_bar(self, context):
        yield 'FOO_BAR', context['FOO'] + context['BAR']

All you need to do is:

• Inherit from AutoPreset

• Put your code in methods named pre_* or post_* (based on when you want to execute your code)

• Yield all tuples of (key, value) that you want to define in the configuration

Optionally you can ask for env or context in your arguments and they will be injected for you.