essentials-configuration 0.0.2

Implementation of key-value pair based configuration for Python applications.

Python configuration utilities

Implementation of key-value pair based configuration for Python applications.

Features:

• support for most common sources of application settings
• support for overriding settings in sequence
• support for nested structures and lists, using attribute notation
• strategy to use environment specific settings

This library is freely inspired by .NET Core Microsoft.Extensions.Configuration namespace and its pleasant design (ref. MSDN documentation, Microsoft Extensions Configuration Deep Dive).

The main class is influenced by Luciano Ramalho`s example of JSON structure explorer using attribute notation, in his book Fluent Python.

Overview

essentials-configuration provides a way to handle configuration roots composed of different layers, such as configuration files and environmental variables. Layers are applied in order and can override each others' values, enabling different scenarios like configuration by environment and system instance.

Supported sources:

• yaml files
• json files
• ini files
• environmental variables
• dictionaries
• keys and values
• Azure Key Vault, using essentials-configuration-keyvault
• custom sources, implementing the ConfigurationSource interface

Installation

pip install essentials-configuration

To install with support for YAML configuration files:

pip install essentials-configuration[yaml]

Extensions

• Azure Key Vault secrets configuration source: essentials-configuration-keyvault

Examples

JSON file and environmental variables

In this example, configuration values will include the structure inside the file settings.json and environmental variables whose name starts with "APP_". Settings are applied in order, so environmental variables with matching name override values from the json file.

from configuration.common import ConfigurationBuilder
from configuration.json import JSONFile
from configuration.env import EnvironmentVariables

builder = ConfigurationBuilder(
    JSONFile("settings.json"),
    EnvironmentVariables(prefix="APP_")
)

config = builder.build()

For example, if the JSON file contains the following contents:

{
    "logging": {
        "level": "INFO"
    },
    "example": "Hello World",
    "foo": "foo"
}

And the environment has a variable named APP_foo=AAA:

>>> config
<Configuration {'logging': '...', 'example': '...', 'foo': '...'}>
>>> config.foo
'AAA'
>>> config.logging.level
'INFO'

YAML file and environmental variables

In this example, configuration will include anything inside a file settings.yaml and environmental variables. Settings are applied in order, so environmental variables with matching name override values from the yaml file (using the yaml source requires also PyYAML package).

from configuration.common import ConfigurationBuilder
from configuration.env import EnvironmentVariables
from configuration.yaml import YAMLFile

builder = ConfigurationBuilder()

builder.add_source(YAMLFile("settings.yaml"))
builder.add_source(EnvironmentVariables())

config = builder.build()

YAML file, optional file by environment

In this example, if an environmental variable with name APP_ENVIRONMENT and value dev exists, and a configuration file with name settings.dev.yaml is present, it is read to override values configured in settings.yaml file.

import os

from configuration.common import ConfigurationBuilder
from configuration.env import EnvironmentVariables
from configuration.yaml import YAMLFile

environment_name = os.environ["APP_ENVIRONMENT"]

builder = ConfigurationBuilder()

builder.add_source(YAMLFile("settings.yaml"))

builder.add_source(YAMLFile(f"settings.{environment_name}.yaml", optional=True))

builder.add_source(EnvironmentVariables(prefix="APP_"))

config = builder.build()

Filtering environmental variables by prefix

from configuration.common import Configuration

config = Configuration()

# will read only environmental variables
# starting with "APP_", case insensitively, removing the "APP_" prefix by
# default
config.add_environmental_variables("APP_")

INI files

INI files are parsed using the built-in configparser module, therefore support [DEFAULT] section; all values are kept as strings.

from configuration.common import ConfigurationBuilder
from configuration.ini import INIFile

builder = ConfigurationBuilder()

builder.add_source(INIFile("settings.ini"))

config = builder.build()

Dictionaries

from configuration.common import ConfigurationBuilder

builder = ConfigurationBuilder()

builder.add_map({"host": "localhost", "port": 8080})

builder.add_map({"hello": "world", "example": [{"id": 1}, {"id": 2}]})

config = builder.build()

assert config.host == "localhost"
assert config.port == 8080
assert config.hello == "world"
assert config.example[0].id == 1
assert config.example[1].id == 2

Keys and values

from configuration.common import ConfigurationBuilder

builder = ConfigurationBuilder()

builder.add_map({"host": "localhost", "port": 8080})

builder.add_value("port", 44555)

config = builder.build()

assert config.host == "localhost"
assert config.port == 44555

Overriding nested values

It is possible to override nested values by environmental variables or dictionary keys using the following notation for sub properties:

• keys separated by colon ":", such as a:d:e
• keys separated by "__", such as a__d__e

from configuration.common import ConfigurationBuilder, MapSource


builder = ConfigurationBuilder(
    MapSource(
        {
            "a": {
                "b": 1,
                "c": 2,
                "d": {
                    "e": 3,
                    "f": 4,
                },
            }
        }
    )
)

config = builder.build()

assert config.a.b == 1
assert config.a.d.e == 3
assert config.a.d.f == 4

builder.add_value("a:d:e", 5)

config = builder.build()

assert config.a.d.e == 5
assert config.a.d.f == 4

Overriding nested values using env variables

import os

builder = ConfigurationBuilder(
    MapSource(
        {
            "a": {
                "b": 1,
                "c": 2,
                "d": {
                    "e": 3,
                    "f": 4,
                },
            }
        }
    )
)

config = builder.build()

assert config.a.b == 1
assert config.a.d.e == 3
assert config.a.d.f == 4

# NB: if an env variable such as:
# a:d:e=5
# or...
# a__d__e=5
#
# is defined, it overrides the value  from the dictionary

os.environ["a__d__e"] = "5"

builder.sources.append(EnvironmentVariables())

config = builder.build()

assert config.a.d.e == "5"

Overriding values in list items using env variables

builder = ConfigurationBuilder(
    MapSource(
        {
            "b2c": [
                {"tenant": "1"},
                {"tenant": "2"},
                {"tenant": "3"},
            ]
        }
    )
)

builder.add_value("b2c:1:tenant", "4")

config = builder.build()

assert config.b2c[0].tenant == "1"
assert config.b2c[1].tenant == "4"
assert config.b2c[2].tenant == "3"

Goal and non-goals

The goal of this package is to provide a way to handle configuration roots, fetching and composing settings from different sources, usually happening once at application's start.

The library implements only a synchronous API and fetching of application settings atomically (it doesn't support generators), like application settings fetched from INI, JSON, or YAML files that are read once in memory entirely. An asynchronous API is currently out of the scope of this library, since its primary use case is to fetch configuration values once at application's start.