Class Instance Configurator

A simple configuration library allowing instantiation of classes from config. files.

Configuration files are in JSON format and simply specify the type of the created object and keyword arguments for its construction. The library also supports free-form config. objects for which respective dataclass-like types are automatically created from a meta-class (so that the programmer doesn’t even have to declare them).

Support for remote config. is provided by (optional) use of smart-open.

The library also provides dot-path syntax access to configuration object hierarchy.

Examples

Instantiation of a class from config. file

Assuming you have the following general class:

my_module.py

from confcls import Configurable

class MyClass(Configurable):
    def __init__(self, arg1: str, arg2: int, arg3: list[str], arg4: dict[str, float]):
        self.foo = arg1
        self.bar = arg2
        self.baz = [arg4[name] for name in arg3 if name in arg4]

You can now store its instance configuration in a JSON file:

my_obj.json

{
    "__type__" : "my_module::MyClass",
    "arg1" : "Hello world!",
    "arg2" : 123,
    "arg3" : ["abc", "ghi"],
    "arg4" : {
        "abc" : 0.123,
        "def" : 0.987
    }
}

And instantiate the configured instance, thus:

from my_module import MyClass

my_obj = MyClass.from_config("my_obj.json")

(You may also call confcls.Configurable.from_config directly if you don’t wish to specify and/or import the type.)

In confcls v2, the common plain dot notation of type values changed to the following: my.module.path::My.Class.Path; that is to say, class specification are separated from module specification by ::. This is necessary to support nested classes.

The instantiation does support nesting; your constructor arguments may indeed be other class instances:

my_obj.json

{
    "__type__" : "my_module::Class1",
    "foo" : 123,
    "bar" : {
        "__type__" : "my_module::Class2",
        "arg1" : 345,
        "arg2" : "whatever"
    },
    "baz" : {
        "__type__" : "my_module::Class3",
        "arg" : {
            "__type__" : "my_module::Class2",
            "arg1" : 567,
            "arg2" : "something else"
        }
    }
}

The above configuration instantiates the same object as the following code:

my_obj = Class1(
    foo=123,
    bar=Class2(arg1=345, arg2="whatever"),
    baz=Class3(arg=Class2(arg1=567, arg2="something else)),
)

Dataclass config

You may want to create a (nested) dataclass configuration. The principle is exactly the same as above; you simply define your configuration data classes and instantiate them from configuration:

my_config.py

from dataclasses import dataclass
from confcls import Configurable

@dataclass
class Configuration(Configurable):
    number: int
    fpnum: float = 0.0  # number with default
    item1: Item1
    opt_item2: Item2 = None  # optional instance

@dataclass
class Item1:  # note that nested dataclasses don't even need to be Configurable
    foo: int
    bar: float = 1.0

@dataclass Item2:
    baz: str

Now, your configuration may look like this:

{
    "__type__" : "my_config::Configuration",
    "number" : 123,
    "item1" : {
        "__type__" : "my_config::Item1",
        "foo" : 345,
        "bar" : 0.5
    }
}

Automatic dataclass-like instances

If you’re very lazy, you don’t even have to declare your config. dataclasses. Just let confcls create the types for you, on demand:

my_config.json

{
    "__type__" : "confcls::Configuration",
    "myarg1" : "whatever you like",
    "myarg2" : {
        "__type__" : "confcls::Object",
        "absolutely" : "anything",
        "really" : 123
    }
}

confcls.Object is a free-form class which can be instantiated with any keyword arguments (and the instance contains them as members). So now, you can access your configuration e.g. like this:

from confcls import Configuration  # Configurable extension of confcls.Object

config = Configuration.from_config("my_config.json")
assert config.myarg2.absolutely == "anything"

Note that this sort of configuration doesn’t support defaults as there’s nowhere to define them (if you want defaults, just declare your (data)classes with them). Also note that you may combine the declared/free-form approaches (if it makes sense).

Finally, observe that the Configurable.from_config member function has auto_obj parameter (with False default). Setting that parameter to True allows you to omit the type specification in your configuration. In that case, the library will automatically treat all JSON objects in the config. file as if the confcls.Object type was specified.

Dot-path Syntax Access

confcls.DotPath allows config["dot.path.to.attr"] style access to configuration objects hierarchy. Each node on the (dot-separated) path represents object specification; be it by: * Attribute name * dict key * list, tuple or string index or slice * Regular expression matching attribute name or dict key

It is enough to derive the top-level config. object from DotPath.Accessible and the following will be possible (note that confcls.Configuration is of course derived from DotPath.Accessible):

my_val = config["foo.bar.baz"]              # same as config.foo.bar.baz
my_val = config["items.3.foo"]              # same as config.items[3].foo
my_val = config.get("foo.bar")              # on key miss, None is returned
my_val = config.get("foo.bar", 123)         # on key miss, the default is returned
my_val = config.items("foo.bar.*")          # iterator of all config.foo.bar attributes
my_val = config.items("foo.r'ba.*'.2:-2")   # config.foo.(all matching attrs).slice(2,-2)

1. and so on…​

Development

To develop confcls, you shall need make, pyenv and poetry installed. Then, setup and initialise your development environment:

$ make setup
$ make init

To run mypy type checks, unit tests, and the linter use the mypy, test and lint make targets, respectively. Alternatively, use the check target to do all that.

$ make check

The Python package is built by the build target. The package may be published using the publish target.

$ make build
$ make publish

Author

Václav Krpec <vencik@razdva.cz>