Settings Collector

This Python 3.6+ package is meant to be used by other packages (i.e., not directly by final projects). Its purpose is to collect settings defined in various frameworks, so you don't have to make special implementations for those.

A typical use case would be to define something like this:

from settings_collector import SettingsCollector, SC_Setting

class my_settings(SettingsCollector):
    foo = SC_Setting("bar")

and then use my_settings.foo in your code. The value of foo would come from Django's settings if your package is used in a Django app, from Flask's app config if it's used in a Flask project, etc.

Content

1. Supported frameworks
2. How to use
3. Settings definitions
4. Prefix
5. Name cases
6. Scopes
7. Fine tuning
8. Local function arguments
9. Settings in projects with no frameworks
10. Custom loaders
11. Testing custom loaders

Supported frameworks

Currently, Settings Collector supports Bottle, CherryPy, Django, Flask, Pyramid, and TurboGears. Please note that the support for most of those was written just by reading their docs. I am experienced only with Django and Flask.

Each framework is handled by its loader class. These are located in the settings_collector.loader package. Apart from the loaders for specific frameworks, there is also a loader for environment variables, called SC_EnvironLoader.

Settings for frameworks are not normally loaded directly from the environment, but they come from some sort of config file (which might import some of them from the environment), so SC_EnvironLoader is disabled by default. To enable it, one can run

from settings_collector import SC_EnvironLoader

SC_EnvironLoader.enabled = True

How to use

The most general way to use this is to define a class similar to the models in ORM frameworks:

from settings_collector import SettingsCollector, SC_Setting

class my_settings(SettingsCollector):
    foo = SC_Setting("bar")

After this, just use my_settings.foo, which will fetch value foo from your project's configuration (which can be standard Django, Flask, etc). If the value is not defined, it will be set to the default value ("bar", in this example).

Note that the loaders adjust character cases as needed. This means that your my_settings.foo will come from django.conf.settings.FOO if your packages is used in a Django app, but from cherrypy.config["foo"] or cherrypy.request.app.config["foo"] if it's used in a CherryPy project. For details, see Name cases.

If one needs to define only the default values for the settings, without further adjustments, this can be used as well:

from settings_collector import SettingsCollector

class my_settings(SettingsCollector):
    defaults = {"bar": "foo"}

This example is equivalent to the previous one, but it doesn't allow adjustments described in the next section.

Settings definitions

The constructor of SC_Setting accepts the following arguments (presented here with their default values):

• default_value [optional]: The default value to be returned if it's not defined in settings. If neither this nor the setting's value in the framework's config is set, a ValueError exception is raised.

• no_cache=False [optional, keyword only]: If True, skip cashing. This means that the actual value of the setting is fetched every time it is requested. Normally, the settings are set up when the app runs and they do not change, so caching is usually the best way to go.

• value_type=None [optional, keyword only]: A type to convert the value to (for example, int). This can be used to ensure the correct type of the value, even if the settings provide something else (for example, a string, as is normal for os.environ).

• default_on_error=True [optional, keyword only]: This affects the behaviour when the value of a setting is set, but the casting to the given value_type fails. If set to True, the returned value is default_value (a ValueError exception is raised if that value is not set). However, if this is set to False, requesting the value not defined in the app will result in TypeError exception, regardless of default_value.

Prefix

If you want your config settings to be distinguished from all others (those used by other packages or the framework itself), you can define a prefix:

from settings_collector import SettingsCollector, SC_Setting

class my_settings(SettingsCollector):
    class SC_Config:
        prefix = "test"
    foo = SC_Setting("bar")

Now, the value of my_settings.foo will come from test__foo in the framework's config.

Name cases

Each loader (the class responsible for the actual loading of the settings' values from the frameworks' configs) can define the case for the names. For example, Django keeps settings in variables that have upper-case names, so the corresponding loader will change your names to upper-case as will the one for os.environ, but not the one for TurboGears.

So, in the previous example, the value of my_settings.foo will come from TEST__FOO if the running framework is Django, but it'll come from test__foo if your package is used in a TurboGears app.

Scopes

Settings can get their values relative to scopes, not unlike names given to loggers. The default separator for scopes' names is double underscore ("__").

Let us take a look at the following Django example. Let's say that the settings are defined as follows (in some framework that uses upper-case names):

FOO = "bar"
SC1__SC2__FOO = "bard"

We then define our settings collector:

from settings_collector import SettingsCollector, SC_Setting

class my_settings(SettingsCollector):
    foo = SC_Setting("bar")

The values obtained will be as follows:

• my_settings.foo == "bar" is defined in the settings as FOO=bar;
• my_settings("sc1").foo == "bar" is inherited from the root scope because there is no foo defined in the scope "sc1" (i.e., there is no SC1__FOO);
• my_settings("sc1__sc2").foo == "bard" is directly defined in the settings as SC1__SC2__FOO = "bard";
• my_settings("sc1__sc2__sc3").foo == "bard" is again inherited from the parent scope "sc1__sc2" because there is no SC1__SC2__SC3__FOO definition.

Fine tuning

Each subclass of SettingsCollector can have a class SC_Config in its definition, which has a similar role to Meta in Django's models: it defines various properties that define the settings collector's behaviour. The available attributes are as follows:

• prefix [default: None]: Explained here.

• sep [default: "__"]: A string containing a separator used between prefix, scopes, and settings' names.

• loaders [default: None]: A sequence of loaders' names, defining the loaders that should be skipped or used (see exclude property). This list can contain loaders that are unknown to the Settings Collector, making it possible to explicitly include or exclude loaders that might be defined only in some frameworks (i.e., outside of settings_collector).

• exclude [default: True]: If True, the loaders listed in loaders are skipped; otherwise, they are the only ones used. Note that the loaders with enabled set to False will always be skipped.

• load_all [default: False]: If False, the settings are loaded by the first loader that can provide them (i.e., the other loaders are not used). If this is changed to True, the values are fetched from all the loaders, with the latter ones overriding the former ones. This'll rarely make sense, since each loader covers one framework and a project is not likely to use more than one of them, but it might make sense if you want to combine settings in the environment variables with those in the framework.

• greedy_load [default: True]: If True, then all the settings are loaded when one of them is requested, thus minimising the overhead of the settings collector. If this is changed to False, each setting is loaded when requested and not before.

Local function arguments

Because Settings Collectors are meant to be used by packages to pull the settings' default values from various frameworks, the packages themselves are not meant to be tied to any specific framework, which means that there could be no framework nor central configuration at all. One would still want their functions and classes to be configurable, and this is normally done through functions' and methods' arguments.

To make it easier to achieve this, the package provides @sc_defaults decorator. Its purpose is to populate functions' and methods' arguments from a settings collector during runtime, assuming that the values were not given in the call nor as function's or method's defaults.

A typical use case is this:

class my_settings(SettingsCollector):

    foo = SC_Setting("food")
    bar = SC_Setting("bard")


@sc_defaults(my_settings)
def f(x, foo):
    return f"{x}:{foo}"

Here, we can do several different calls (assuming that foo and bar are not set in the framework):

• f(17) returns "17:food" because x = 17 comes from the call, while foo is unset and thus gets its value from my_settings.foo.

• f(17, "afoot"), f(17, foo="afoot"), and f(x=17, foo="afoot") all return "17:afoot" because both values were provided in the function call.

The same can be done with methods in classes, with one caveat: if the method in question is a class method, the @classmethod decorator must be put before @sc_defaults.

If some argument has its default defined in the function's or method's signature, its value will never be picked from the attached settings collector.

Scopes are supported by an optional keyword argument scope_arg which holds the name of the argument that defines scope. If not set, the scopes will not be used. Here is an example:

@sc_defaults(my_settings, scope_arg="namespace")
def f(foo, namespace=None):
    return f"{foo}"

print("Argument-provided foo: ", f("foot"))
print("Default scope's foo:   ", f())
print("Value of foo in scope1:", f(namespace="scope1"))

For more usage examples, see tests/test_defaults.py.

Settings in projects with no frameworks

Projects that do not use any of the supported or supporting frameworks can still adjust settings for those packages that support Settings Collector. This is done through a dictionary-like object called sc_settings and is used like this:

from settings_collector import sc_settings

# Set values one by one:
sc_settings["prefix1__some_setting"] = ...
sc_settings["prefix2__scope1__scope2__some_setting"] = ...
...

# Or, in bulk:
sc_settings.update({
    "prefix1__some_setting"]: ...,
    "prefix2__scope1__scope2__some_setting"]: ...,
    ...
}

Note that you should never do sc_setting = .... If you do, the loader responsible for reading this data will rebel by raising a SC_SettingsError exception.

This can be used even without any other packages using Settings Collector, as a placeholder for your project's configuration, although it's a bit questionable what the benefits would be (over just having your own Flask-like config dictionary or Django-like class settings), especially if one does not need scopes.

Custom loaders

Adding the support for Settings Collector to your own framework is easy. Obviously, you can submit it to me for inclusion in this package, but it is just as easy to include it in the package itself.

All you need to do is create a module:

try:
    from settings_collector import SC_LoaderBase
except ImportError:
    pass
else:
    class SC_MyFrameworkLoader(SC_LoaderBase):
        ...

The name of your loader class must begin with SC_ and end with Loader.

Make sure that this module is imported. The creation of a subclass of SC_LoaderBase automatically registers it with the one that Settings Collector can recognise.

Notice that this module does not require settings_collector, so your framework won't require it either. If no packages requiring settings_collector are used, the import above will be silently ignored.

Since the configurations are usually defined as attributes in modules or classes (like in Django) or as keys in a dictionary (like in Flask or os.environ), there are subclasses that make it easier to implement loaders for any framework using one of these approaches. These are SC_LoaderFromAttribs and SC_LoaderFromDict. To see how they are used, see the source code for SC_DjangoLoader and for SC_EnvironLoader.

Testing custom loaders

One can easily test their shiny new loader.

1. Install Settings Collector in some project using your framework:

   pip install settings_collector
   

2. Add your loader to the project and import its module (so that the loader gets registered).

3. Run the following code:

   import settings_collector
   settings_collector.sc_test_print_expected()
   

   This will print the testing variables as the test function expects them.

4. Add these settings to your project's config. Make sure that they are named consistently with how it is done in your framework. For example, the names of these settings would be uppercased in Django's config, despite them having mixed cases in the output of sc_test_print_expected.

5. Restart your project and run:

    import settings_collector
    settings_collector.sc_test_run()
   

Note that you can also run settings_collector.sc_test_run(False) if you want a bit less verbose output.