How can I set a default value for missing environment variables?

[brianbruggeman]

Also here: https://stackoverflow.com/questions/42982267/tox-set-default-values-for-missing-environment-variables

I've found that I would really like to use environment variables to control flow - especially when testing. But I also would like to set a default behavior based on whether or not an environment variable is present. In bash, I'd just use: ${ENV_VAR:-default_value}, but I don't have that basic option in Tox (unless I missed it).

Is there a way?

[obestwalter]

This works via the env substitution. An example for overriding the toxworkdir with a default:

tox.ini

[tox]
envlist = py
skipsdist = true

[testenv]
setenv = TOX_WORKDIR = {env:TOX_WORKDIR:.tox}
commands = python ./envecho.py

envecho.py

#!/usr/bin/env python
import os

print(os.getenv("TOX_WORKDIR"))

output without env set:

$ tox
       
py create: /home/oliver/Dropbox/projects/tox/lab/mini/.tox/py
py installed: appdirs==1.4.3,packaging==16.8,pyparsing==2.2.0,six==1.10.0
py runtests: PYTHONHASHSEED='2704413679'
py runtests: commands[0] | python ./envecho.py
.tox
______________________________ summary ______________________________
  py: commands succeeded
  congratulations :)

output with env set:

$ TOX_WORKDIR=/tmp/ tox
       
py installed: appdirs==1.4.3,packaging==16.8,pyparsing==2.2.0,six==1.10.0
py runtests: PYTHONHASHSEED='30398946'
py runtests: commands[0] | python ./envecho.py
/tmp/
______________________________ summary ______________________________
  py: commands succeeded
  congratulations :)

[obestwalter]

It's dawning on me, that you might want to set the value of the environment variable itself if it is not set from the outside ... is that right?

[brianbruggeman]

Yes.

[brianbruggeman]

But I'm wondering if I can do something fancy...

[tox-env-identifier]
...
setenv
    VAR_NAME = {env:VAR_NAME:default_value}
...

[obestwalter]

Fixed the answer after understanding the question properly :)

[obestwalter]

Correct :)

[brianbruggeman]

Can I do something like this?

setenv =
    SPARK_PYTHONPATH = {env:SPARK_PYTHONPATH:env:SPARK_HOME/python}
    PYTHONPATH = {env:SPARK_PYTHONPATH}:{toxinidir}

[obestwalter]

setenv =
SPARK_PYTHONPATH = {env:SPARK_PYTHONPATH:env:SPARK_HOME/python}

AFAIK you can not access the content of env vars in setenv, you can only set them.

PYTHONPATH = {env:SPARK_PYTHONPATH}:{toxinidir}

I think the correct syntax would be PYTHONPATH = {env:SPARK_PYTHONPATH:{toxinidir}} but when I try this I get: {env:HOME:/home/oliver/Dropbox/projects/tox/lab/mini} as output of the env var, so the current substtution engine cannot handle that (or I am getting something wrong also).

If you start doing magic like this you can always add a thin wrapper to call your tests and do the extra magic there. This is not super elegant but that way you have full flexibility. You can take an ini file with some substitution magic only so far without creating a hideous monster.

[brianbruggeman]

In the case of the PYTHONPATH variable above, it already does explicitly what I want (concatenates SPARK_PYTHONPATH with the current toxini folder). So I wasn't too concerned there.

But SPARK_PYTHONPATH is the real issue as it requires knowledge of the SPARK_HOME value to set a valid default value. I completely empathize with trying to support recursive/layered structures like this... And the truth is that I'm not sure any of the default configuration technologies really handle this kind of scenario: ini, json, yaml, toml, etc. But I'm finding this pattern more common now, and I'm going to need to figure out a better solution for it. Requiring environment variables set to a specific value is not a great one.

Thank you for your help here!