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A Neat configuratioN Auxiliary

Project Description

Anna helps you configure your application by building the bridge between the components of your application and external configuration sources. It allows you to keep your code short and flexible yet explicit when it comes to configuration - the necessary tinkering is performed by the framework.

Anna contains lots of “in-place” documentation aka doc strings so make sure you check out those too (“help yourself”)!

80 seconds to Anna

Anna is all about parameters and configuration sources. You declare parameters as part of your application (on a class for example) and specify their values in a configuration source. All you’re left to do with then is to point your application to the configuration source and let the framework do its job.

An example is worth a thousand words

Say we want to build an application that deals with vehicles. I’m into cars so the first thing I’ll do is make sure we get one of those:

>>> class Car:
...     def __init__(self, brand, model):
...         self._brand = brand
...         self._model = model
>>> your_car = Car('Your favorite brand', 'The hottest model')

Great! We let the user specify the car’s brand and model and return him a brand new car!

Now we’re using anna for declaring the parameters:

>>> from anna import Configurable, parametrize, String, JSONAdaptor
>>> @parametrize(
...     String('Brand'),
...     String('Model')
... )
... class Car(Configurable):
...     def __init__(self, config):
...         super(Car, self).__init__(config)
>>> your_car = Car(JSONAdaptor('the_file_where_you_specified_your_favorite_car.json'))

The corresponding json file would look like this:

    "Car/Parameters/Brand": "Your favorite brand",
    "Car/Parameters/Model": "The hottest model",

It’s a bit more to type but this comes at a few advantages:

  • We can specify the type of the parameter and anna will handle the necessary conversions for us; anna ships with plenty of parameter types so there’s much more to it than just strings!
  • If we change your mind later on and want to add another parameter, say for example the color of the car, it’s as easy as declaring a new parameter String('Color') and setting it as a class attribute; all the user needs to do is to specify the corresponding value in the configuration source. Note that there’s no need to change any interfaces/signatures or other intermediate components which carry the user input to the receiving class; all it expects is a configuration adaptor which points to the configuration source.
  • The configuration source can host parameters for more than only one component, meaning again that we don’t need to modify intermediate parts when adding new components to our application; all we need to do is provide the configuration adaptor.

Five minutes hands-on

The 80 seconds intro piqued your curiosity? Great! So let’s move on! For the following considerations we’ll pick up the example from above and elaborate on it more thoroughly.

Let’s start with a quick Q/A session

So what happened when using the decorator ``parametrize``? It received a number of parameters as arguments which it set as attributes on the receiving class. Field names are deduced from the parameters names applying CamelCase to _snake_case_with_leading_underscore conversion. That is String('Brand') is set as Car._brand.

All right, but how did the instance receive its values then? Note that Car inherits from Configurable and Configurable.__init__ is where the actual instance configuration happens. We provided it a configuration adaptor which points to the configuration source (in this case a local file) and the specific values were extracted from there. Values are set on the instance using the parameter’s field name, that is String('Brand') will make an instance receive the corresponding value at your_car._brand (Car._brand is still the parameter instance).

Okay, but how did the framework know where to find the values in the configuration source? Well there’s a bit more going on during the call to parametrize than is written above. In addition to setting the parameters on the class it also deduces a configuration path for each parameter which specifies where to find the corresponding value in the source. The path consists of a base path and the parameter’s name: “<base-path>/<name>” (slashes are used to delimit path elements). parametrize tries to get this base path from the receiving class looking up the attribute CONFIG_PATH. If it has no such attribute or if it’s None then the base path defaults to “<class-name>/Parameters”. However in our example - although we didn’t set the config path explicitly - it was already there because Configurable uses a custom metaclass which adds the class attribute CONFIG_PATH if it’s missing or None using the same default as above. So if you want to specify a custom path within the source you can do so by specifying the class attribute CONFIG_PATH.

_snake_case_with_leading_underscore, not too bad but can I choose custom field names for the parameters too? Yes, besides providing a number of parameters as arguments to parametrize we have the option to supply it a number of keyword arguments as well which represent field_name / parameter pairs; the key is the field name and the value is the parameter: brand_name=String('Brand').

Now that we declared all those parameters how does the user know what to specify? anna provides a decorator document_parameters which will add all declared parameters to the component’s doc string under a new section. Another option for the user is to retrieve the declared parameters via get_parameters (which is inherited from Configurable) and print their string representations which contain comprehensive information:

>>> for parameter in Car.get_parameters():
...     print(parameter)

Of course documenting the parameters manually is also an option.

Alright so let’s get to the code

>>> from anna import Configurable, parametrize, String, JSONAdaptor
>>> @parametrize(
...     String('Model'),
...     brand_name=String('Brand')
... )
... class Car(Configurable):
...     CONFIG_PATH = 'Car'
...     def __init__(self, config):
...         super(Car, self).__init__(config)

Let’s first see what information we can get about the parameters:

>>> for parameter in Car.get_parameters():
...     print(parameter)
    "optional": false,
    "type": "StringParameter",
    "name": "Model",
    "path": "Car"
    "optional": false,
    "type": "StringParameter",
    "name": "Brand",
    "path": "Car"

Note that it prints "StringParameter" because that’s the parameter’s actual class, String is just a shorthand. Let’s see what we can get from the doc string:

>>> print(Car.__doc__)
>>> from anna import document_parameters
>>> Car = document_parameters(Car)
>>> print(Car.__doc__)

    Declared parameters
    (configuration path: Car)

    Brand : String
    Model : String

Now that we know what we need to specify let’s get us a car! The JSONAdaptor can also be initialized with a dict as root element, so we’re just creating our configuration on the fly:

>>> back_to_the_future = JSONAdaptor(root={
...     'Car/Brand': 'DeLorean',
...     'Car/Model': 'DMC-12',
... })
>>> doc_browns_car = Car(back_to_the_future)
>>> doc_browns_car.brand_name  # Access via our custom field name.
>>> doc_browns_car._model  # Access via the automatically chosen field name.

Creating another car is as easy as providing another configuration source:

>>> mr_bonds_car = Car(JSONAdaptor(root={
...     'Car/Brand': 'Aston Martin',
...     'Car/Model': 'DB5',
... }))

Let’s assume we want more information about the brand than just its name. We have nicely stored all information in a database:

>>> database = {
... 'DeLorean': {
...     'name': 'DeLorean',
...     'founded in': 1975,
...     'founded by': 'John DeLorean',
... },
... 'Aston Martin': {
...     'name': 'Aston Martin',
...     'founded in': 1913,
...     'founded by': 'Lionel Martin, Robert Bamford',
... }}

We also have a database access function which we can use to load stuff from the database:

>>> def load_from_database(key):
...     return database[key]

To load this database information instead of just the brand’s name we only have to modify the Car class to declare a new parameter: ActionParameter (or Action). An ActionParameter wraps another parameter and let’s us specify an action which is applied to the parameter’s value when it’s loaded. For our case that is:

>>> from anna import ActionParameter
>>> Car.brand = ActionParameter(String('Brand'), load_from_database)
>>> doc_browns_car = Car(back_to_the_future)
>>> doc_browns_car.brand
{'founded by': 'John DeLorean', 'name': 'DeLorean', 'founded in': 1975}
>>> doc_browns_car.brand_name

Note that we didn’t need to provide a new configuration source as the new brand parameter is based on the brand name which is already present.

Say we also want to obtain the year in which the model was first produced and we have a function for exactly that purpose however it requires the brand name and model name as one string:

>>> def first_produced_in(brand_and_model):
...     return {'DeLorean DMC-12': 1981, 'Aston Martin DB5': 1963}[brand_and_model]

That’s not a problem because an ActionParameter type lets us combine multiple parameters:

>>> Car.first_produced_in = ActionParameter(
... String('Brand'),
... lambda brand, model: first_produced_in('%s %s' % (brand, model)),
... depends_on=('Model',))

Other existing parameters, specified either by name of by reference via the keyword argument depends_on, are passed as additional arguments to the given action.

In the above example we declared parameters on a class using parametrize but you could as well use parameter instances independently and load their values via load_from_configuration which expects a configuration adaptor as well as a configuration path which localizes the parameter’s value. You also have the option to provide a specification directly via load_from_representation. This functions expects the specification as a unicode string and additional (meta) data as a dict (a unit for PhysicalQuantities for example).

This introduction was meant to demonstrate the basic principles but there’s much more to anna (especially when it comes to parameter types)! So make sure to check out also the other parts of the docs!

Parameter types

A great variety of parameter types are here at your disposal:

  • Bool
  • Integer
  • String
  • Number
  • Vector
  • Duplet
  • Triplet
  • Tuple
  • PhysicalQuantity
  • Action
  • Choice
  • Group

Configuration adaptors

Two adaptor types are provided:

  • XMLAdaptor for connecting to xml files.
  • JSONAdaptor for connecting to json files (following some additional conventions).

Generating configuration files

Configuration files can of course be created manually however anna also ships with a PyQt4 frontend that can be integrated into custom applications. The frontend provides input forms for all parameter types as well as for whole parametrized classes together with convenience methods for turning the forms’ values into configuration adaptor instances which in turn can be dumped to files. See anna.frontends.qt.

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