stela 1.0.4

Find and read your project configuration files easily

Welcome to Stela

Stela were the "configuration files" of ancient times. This library aims to simplify your project configurations, from json, yaml, ini or toml files in a single Python dictionary, which values can be easily be override by environment variables.

Install

$ pip install stela

Example

You can use .yaml, .ini, .json and .toml files. Suppose a file called development.ini which contains:

[foo]
bar = value

Stela will convert data into a python dictionary:

{
    "foo": {
        "bar": "value"
    }
}

And you can use the settings like this:

from stela import settings

my_conf = settings["foo.bar"]
# my_conf = "value"

This is possible because Stela uses under the hood the Scalpl library.

How Stela find the configuration files?

By default, Stela will use the value from ENVIRONMENT environment variable to find correspondent INI file. For example, if you set ENVIRONMENT=development Stela will look for development.ini file in project root.

You can change this behaviour inside pyproject.toml file:

[tools.stela]
environment_variable_name = "ENVIRONMENT"
config_file_extension = "INI" # YAML, TOML, JSON
config_file_prefix = ""  # You can add a prefix before name - ex.: env_development.ini
config_file_suffix = ""  # You can add a suffix after name - ex.: development_v1.ini
default_environment = "development" # use this if you do not want to define the ENVIRONMENT key
config_file_path = "."  # relative path from project root for configuration files

How Stela find the Environment Variables?

When asked for a value, Stela will try to find a environment variable using the full uppercase slug path. For example, if key is foo.bar, Stela will search for a env called FOO_BAR before returning his dict value. Also, you can add a prefix/suffix in this calculated environment name (ex.: MYPROJECT_FOO_BAR). To do this, define them in pyproject.toml:

[tools.stela]
environment_prefix = "MYPROJECT_"
environment_suffix = ""

In above case, Stela will look for the MYPROJECT_FOO_BAR env:

# FOO_BAR = "hello_world" or
# MYPROJECT_FOO_BAR = "hello world" if you define environment_prefix
from stela import settings

my_conf = settings["foo.bar"]
# my_conf = "hello world"

Also, you can define Stela to never get values from environment variables, only from dictionary:

[tools.stela]
do_not_read_environment = true

How Stela handle more complex cases?

Stela uses this lifecycle to handle the settings load:

Pre-Load (optional) -> Load or Default Load > Post-Load (optional)

If you have more complex cases to retrieve your project settings (ex.: reading external services, load settings from database or parse from a different file format or library), you can use Stela decorators for pre_load, load and post_load phases:

• If you use the pre_load decorator, data parsed in this phase will be passed to Load phase.
• If you use the load decorator, it will be used instead the Stela's default_loader. The data returned here will update the dictionary received in previous phase
• If you use the post_load decorator, data returned in this phase will update the dictionary received in previous phase.

Only one function are allowed per phase.

These loaders need to return a valid Python dictionary.

Lifecycle example:

from stela.decorators import pre_load, load, post_load
from typing import Dict, Any
from stela import StelaOptions

@pre_load
def pre_load(options: StelaOptions) -> Dict[Any, Any]:
    # data value is: {}
    # Stela Options are available in options object.
    return {"foo": "bar"}

@load
def load(data: dict, options: StelaOptions) -> Dict[Any, Any]:
    # data value is: {"foo": "bar"}
    # Using load, default_loader will not be invoked
    return {"has_dogs": True}

@post_load
def post_load(data: dict, options: StelaOptions) -> Dict[Any, Any]:
    # data value is: {"foo": "bar", "has_dogs": True}
    return {"number_of_dogs": 1}

# Final data is {"foo": "bar", "has_dogs": True, "number_of_dogs": 1}

Post-Load SSM Example:

# stela_loader.py
import boto3
from stela.decorators import post_load
from stela import StelaOptions
from typing import Dict, Any

@post_load
def add_ssm_parameters(data: dict, options: StelaOptions) -> Dict[Any, Any]:
    """Load settings from SSM to current Stela data.
    
    This loader will be called after Stela default loader.
    Data returned must be a Python Dictionary.
    
    :param data (dict): Data parsed from previous phases
    :param options (obj): Stela Options from pyproject.toml
    :return Dict[Any, Any]
    """
    ssm = boto3.client('ssm')
    environment = options.current_environment
    parameters = ssm.get_parameters_by_path(
        Name=f'/foo/bar/{environment}',
        WithDecryption=True
    )
    return parameters

When Stela read the data?

Stela are imported once, at module level. This is the python equivalent for a Singleton object - project settings are load and immediately available:

from stela import settings
from flask import Flask

app = Flask(__name__)
app.config.update(
    SECRET_KEY=settings["my_app.secret"] # will read from dict or MY_APP_SECRET value
)

@app.route("/")
def hello():
    return f"Hello, Environment is {settings.stela_options.current_environment}"

If you need to reload settings, use the stela.stela_reload function:

from stela import stela_reload

def test_different_environments(monkeypatch):
    from stela import settings
    assert settings.stela_options.current_environment == "test"
    
    monkeypatch.setenv("ENVIRONMENT", "production")
    settings = stela_reload()
    assert settings.stela_options.current_environment == "production"
    monkeypatch.delenv("ENVIRONMENT")

How Stela read the dictionary values?

Stela will respect the file format limitations. For example, INI files always return values as string, TOML files returning datetime objects, etc...

For environment variables, Stela will return value as string, by default. For example: NUMBER_OF_CATS=3 will return a string.

You can set Stela to literal evaluate these values, as per ast.literal_eval rules. To do this, add in pyproject.toml:

[tool.stela]
evaluate_data = true

All Stela Configuration Options:

All configuration files can be override using a environment variable, using the STELA_ prefix. For example, default_environment turns STELA_DEFAULT_ENVIRONMENT.

# All keys and default values available in pyproject.toml
[tool.stela]
environment_variable_name = "ENVIRONMENT"
config_file_extension = "INI" # YAML, TOML, JSON
config_file_prefix = ""  # You can add a prefix before name - ex.: env_development.ini
config_file_suffix = ""  # You can add a suffix after name - ex.: development_v1.ini
config_file_path = "."
environment_prefix = ""  # ex.: settings["foo.bar"'] looks for MY_PREFIX_FOO_BAR
environment_suffix = ""  # ex.: settings["foo.bar"'] looks for FOO_BAR_MY_SUFFIX
default_environment = ""
evaluate_data = false
do_not_read_environment = false
show_logs = true  # as per loguru settings.

Not working?

Dont panic. Get a towel and, please, open a issue.