wr-profiles 2.1.0

Switch environment variable based profiles in Python with ease

## Disclaimer

wr- in the package name stands for Wheel Reinvented. Just like all other packages whose name starts with wr-, the meaning of this library (as in meaning of life) lies in its existence and evolution and not in any external application of it.

## Why Do I Need This?

You don’t.

But you could find it useful if you use environment variables as primary means of passing configuration to your program, and you have scenarios when your program has to switch between sets of environment variables.

## Example

Here’s an example of declaring a profile with three properties and using it to consult the values of these properties. More advanced examples are available in the user guide below.

This example works in Python 3.6+. In Python 3.5 you need to pass the name of the property to the Property initialiser: password = Password(‘password’).

import os from wr_profiles import Profile, Property

class WarehouseProfile(Profile):

profile_root = ‘warehouse’ username = Property(default=’default-username’) password = Property()

# export WAREHOUSE_PROFILE=staging # export WAREHOUSE_STAGING_PARENT_PROFILE=”production” # export WAREHOUSE_STAGING_PASSWORD=”staging-password” # export WAREHOUSE_PRODUCTION_USERNAME=”production-username” # export WAREHOUSE_PRODUCTION_PASSWORD=”production-password”

warehouse_profile = WarehouseProfile() assert warehouse_profile.username == ‘production-username’ assert warehouse_profile.password == ‘staging-password’

## Installation

pip install wr-profiles

If you decide to use this library, make sure you pin the version number in your requirements file.

## User Guide

### Concepts

##### Profile

A profile represents a set of configurable properties of a single service backed by environment variables.

There can be multiple unrelated profiles (multiple classes extending Profile class), each providing interface to properties of a different service.

Instances of profiles associated with the same service share the same base class and are identified by profile_root specified in that base class. Is is the root from which all relevant environment variable names are formed.

Profiles of unrelated services do not share any information. In the discussion below, different instances or kinds of profiles all relate to the same service, e.g. same profile_root.

##### Warehouse Profile (Example)

In the discussion below, we will use a profile for a data warehouse access as an example. Class WarehouseProfile declares the profile and the properties it provides. Object warehouse_profile is the single instance through which user must look up service’s active configuration.

class WarehouseProfile(Profile):

profile_root = ‘warehouse’

host = Property(default=’localhost’) username = Property() password = Property(default=’’)

warehouse_profile = WarehouseProfile()

##### Profile Name

Individual instances of profiles are identified by their name (profile_name property).

##### Active Profile

The active profile is the profile of a service that should be used according to the environment variables.

The active profile can be switched by setting a special environment variable <PROFILE_ROOT>_PROFILE. For WarehouseProfile that would be WAREHOUSE_PROFILE.

If this variable is not set, the active profile consults environment variables in the form:

<PROFILE_ROOT>_<PROPERTY_NAME>

For example, WAREHOUSE_HOST.

If <PROFILE_ROOT>_PROFILE is set then the active profile consults environment variables in the form:

<PROFILE_ROOT>_<PROFILE_NAME>_<PROPERTY_NAME>

For example, if WAREHOUSE_PROFILE is set to staging then host property will be looked up under WAREHOUSE_STAGING_HOST.

##### Parent Profile

Any particular profile (for example, staging profile of WarehouseProfile) can be instructed to inherit its property values from a parent profile by setting:

<PROFILE_ROOT>_<PROFILE_NAME>_PARENT_PROFILE

For example, WAREHOUSE_STAGING_PARENT_PROFILE, if set to production, would mean that if environment variable WAREHOUSE_STAGING_HOST was not set, property value loader would consult WAREHOUSE_PRODUCTION_HOST instead. And only if that variable was not present, the default value of the property (if available) would be used.

Limitation: The default profile (profile_name=””) cannot be used as a parent profile. If you specify empty string as <PROFILE_ROOT>_<PROFILE_NAME>_PARENT_PROFILE then this profile won’t have any parent profile. It is the same as having no value set.

##### Live Profile vs Frozen Profile

A live profile always consults environment variables (os.environ) whereas a frozen profile does so only during instantiation and when explicitly loaded with load() method.

### Scenarios

##### Current Active Profile

Current active profile is always available through the instance of your profile class which is instantiated with no arguments:

warehouse_profile = WarehouseProfile()

Normally you’d only need a single instance of your profile class.

##### Concrete Profile

To work with a concrete profile which may not necessarily be activated, use get_instance factory method:

staging = WarehouseProfile.get_instance(‘staging’)

By default, this profile will be frozen which means it will be loaded only once during instantiation. If you want it to always consult environment variables, pass is_live=True:

staging = WarehouseProfile.get_instance(‘staging’, is_live=True)

##### Activate Profile

To activate a profile, call activate method on a frozen instance of the profile without any arguments, or, activate(profile_name) on the live current profile instance:

staging.activate()

# or:

warehouse_profile.activate(‘staging’)