levy 0.4

Dynamic python configuration parser

levy

Yet Another Configuration Parser

This project is a lightweight take on configuration parsing with a twist.

So far, it only supports YAML files or reading configurations directly from a dict.

The interesting approach here is regarding handling multiple environments. Usually we need to pass different parameters depending on where we are (DEV, PROD, and any arbitrary environment name we might use). It is also common to have these specific parameters available as env variables, be it our infra or in a CI/CD process.

levy adds a jinja2 layer on top of our YAML files, so that not only we can load env variables on the fly, but helps us leverage templating syntax to keep our configurations centralized and DRY.

How to

Let's suppose we have the following configuration:

title: "Lévy the cat"
colors:
  - "black"
  - "white"
hobby:
  eating:
    what: "anything"
friends:
  {% set friends = [ "cartman", "lima" ] %}
  {% for friend in friends %}
  - name: ${ friend }
    type: "cat"
  {% endfor %}

We have a bit of everything:

• Root configurations
• Simple lists
• Nested configurations
• Dynamic jinja2 lists as nested configurations

We can create our Config object as

from levy.config import Config

cfg = Config.read_file("test.yaml")

As there is the jinja2 layer we might want to check what is the shape of the parsed values. We can do so with cfg._vars. In our case we'll get back something like:

{
'title': 'Lévy the cat',
'colors': ['black', 'white'],
'hobby': {
  'eating': {
    'what': 'anything'
    }
  },
'friends': [
  {'name': 'cartman', 'type': 'cat'},
  {'name': 'lima', 'type': 'cat'}
  ]
}

OBS: When reading from files and for debugging purposes, we can access the cfg._file var to check what file was parsed.

Accessing values

All the information has been set as attributes to the Config instance. We can retrieve the values as cfg.<name>, e.g.

cfg.title  # 'Lévy the cat'
cfg.colors  # ['black', 'white']

Note that so far those are just root values, as they come directly from the root configuration. Whenever we have a nested item, we are creating a Config attribute with the key as name:

print(cfg)  # Config(root)
print(cfg.hobby)  # Config(hobby)

If we need to retrieve nested values, as we are just nesting Config instances, we can keep chaining attribute calls:

cfg.hobby.eating.what  # 'anything'

Nested Config lists

The colors list has nothing fancy in it, as we have simple types. However, we want to parse nested configurations as Config, while being able to access them by name as attributes.

To fit this spot we have namedtuples. The list attribute becomes a namedtuple where the properties are the names of the nested items. name is set as the default identifier, but we can pass others as parameter,

print(cfg.friends.lima)  # Config(lima)
cfg.friends.lima.type  # 'cat'

And if we check the type...

isinstance(cfg.friends, tuple)  # True

If we encounter an error while defining the namedtuples structure, we will get a ListParseException. We should then check how are we defining the lists and our list_id.

OBS: Note that the list_id field should be a valid namedtuple key. This means that it cannot contain spaces or other not supported special characters.

Using defaults

It is common to fall back to default values when some parameter is not informed in our configuration.

We can __call__ our Config in order to be able to apply them.

cfg("not in there", default="default")  # 'default'
cfg("not in there", default=None)  # None

If no default is specified, the call will run the usual attribute retrieval. This is interesting for cases where we need to dynamically get some configuration that should be there:

cfg("not in there")  # AttributeError

Render custom functions

Environment Variables

With this templating approach on top of YAML, we can not only use default behaviors, but also define our own custom functionalities.

The one we have provided by default is reading environment variables at render time:

variable: ${ env('VARIABLE') }
default: ${ env('foo', default='bar') }

Where the function env is the key name given to a function defined to get env vars with an optional default. If the env variable is not found and no default is provided, we'll get a MissingEnvException.

Registering new functions

If we need to apply different functions when rendering the YAML, we can register them by name before instantiating the Config class.

Let's imagine the following YAML file:

variable: ${ my_func(1) }
foo: ${ bar('x') }

We then need to define the behavior of the functions my_func and bar.

from levy.config import Config
from levy.renderer import render_reg

@render_reg.add()  # By default, it registers the function name
def my_func(num: int):
    return num + 1

@render_reg.add('bar')  # Name can be overwritten if required
def upper(s: str):
    return s.upper()

cfg = Config.read_file("<file>")
cfg.variable  # 2
cfg.foo  # 'X'

Note how we registered my_func with the same name it appeared in the YAML. However, the name is completely arbitrary, and we can pass the function upper with the name bar.

With this approach one can add even further dynamism to the YAML config files.

To peek into the registry state, we can run:

render_reg.registry

Which in the example will show us

{'env': <function __main__.get_env(conf_str: str, default: Optional[str] = None) -> str>,
 'my_func': <function __main__.my_func(num: int)>,
 'bar': <function __main__.upper(s: str)>}

Schema Validation

At some point it might be interesting to make sure that the YAML we are reading follows some standards. That is why we have introduced the ability to pass a schema our file needs to follow.

The schema validation is done using the module jsonschema, which follows the JSON schema specification.

As an example, if we want to make sure our kitten data is valid, we can write:

schema = {
    "type": "object",
    "properties": {
        "title": {"type": "string"},
        "colors": {"type": "array", "items": {"type": "string"}},
        "hobby": {
            "type": "object",
            "properties": {
                "eating": {
                    "type": "object",
                    "properties": {
                        "what": {"type": "string"}
                    }
                }
            }
        },
        "friends": {"type": "array", "items": {"type": "object"}}
    }
}

Config.read_file("<file>", schema=schema)  # this should run OK

In case the validation fails, we'll get a ValidationError. If the schema is incorrect, the exception will be a SchemaError instead.

Contributing

You can install the project requirements with make install. To run the tests, make install_test and make unit.

With make precommit_install you can install the pre-commit hooks.

To install the package from source, clone the repo, pip install flit and run flit install.

References

• pyconfs as inspiration.