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e2e.env

Class-based modeling of environment variables and their type conversions.

Especially suited to test automation where environment variables are used in abundance, or even in applications that could benefit from these mappings.

Requirements

For ease of implementation, Python 3.6 is required in order to use PEP-526 variable annotations.

Overview

Environment variable access is common, and usually done via a module-based approach.

SERVICE_HOST = os.getenv('COMPANY_APP_HOST')
SERVICE_PORT = int(os.getenv('COMPANY_APP_PORT'))

This works just fine, but sometimes they need to be refreshed...

# Whoops, forgot the int conversion!
SERVICE_PORT = os.getenv('COMPANY_APP_PORT')

... which isn't very DRY.

Additionally, you may sometimes need to keep track of the name and the value from the environment,

# This is getting verbose quickly
ENVNAME_SERVICE_HOST = 'COMPANY_APP_HOST'
SERVICE_HOST = os.getenv(ENVNAME_SERVICE_HOST)
ENVNAME_SERVICE_PORT = 'COMPANY_APP_PORT'
SERVICE_PORT = int(os.getenv(ENVNAME_SERVICE_PORT))

In one swoop, we can map the environment variables as well as their conversions to native types, and fetch either the value or the name:

class ServiceVars(e2e.env.EnvMapper):
    host: str = 'COMPANY_APP_HOST'
    port: int = 'COMPANY_APP_PORT'

# Get the port via instance
print(ServiceVars().port)   # 8080
type(ServiceVars().port)    # <class 'int'>

# Get the name of the port environment variable via class
print(ServiceVars.port)     # COMPANY_APP_PORT
type(ServiceVars.port)      # <class 'str'>

Using your own "converters"

All EnvMapper does is read the type annotation and constructs the returned value by passing the environment value to it.

That is, when modeling,

    mapped_name: annotated_type = 'ENV_VAR_NAME'

... on access via the EnvMapper instance, becomes ...

    annotated_type(os.getenv('ENV_VAR_NAME'))

In the above examples for example, we had port: int = 'COMPANY_APP_PORT'. This essentially gets shuffled into int(os.getenv('COMPANY_APP_PORT')). So any callable that can take a single str in its constructor and return the appropriate type will work.

Production use

The code is incredibly simple, and will adhere to these contracts:

  • Variables that do not exist will cause a e2e.env.exceptions.NoSuchVariableError to be raised.*
  • Access of an unmapped environment variable will raise an AttributeError, as would be reasonably expected.
  • Access of a mapping without an annotation will raise a TypeError with the mapping name and model class.

* Open for discussion. Returning None could work. Passing None to the type converter usually won't produce consistent behaviour across types, and so can't be determined as a special case (e.g. str(None) gives "None", int(None) raises a TypeError). See Issue #1 for more info.

Future work

  • Support raise_on_dne or something similar to change what happens when an environment variable is not found. Please add a thumbs-up for Issue #1 if you'd like to see this feature. 
    class ServiceVars(e2e.env.EnvMapper, raise_on_dne=False): ...
    class ServiceVars(e2e.env.EnvMapper, dne=lambda: None): ...
  • Support for combining mappings into one larger mapping, for organizational purposes. Please add a thumbs-up for Issue #2 if you'd like to see this feature.