Render Custom Functions

Environment Variables

It is common to have these specific parameters available as env variables, be it our infra or in a CI/CD process. In these situations, we often end up duplicating configurations, which might end up with issues rising from a configuration drift between environments.

To ease these scenarios levy provides one functionality out of the box: read 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.

We can then access these values as usual.

Registering new functions

If we need to apply different functions when rendering the files, 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>")

// We can access the results of the functions as usual
$ 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 our config files.

Registry

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)>}

OBS: If you take a look at the source code, the env function has been defined in the registry as shown here.