Develop Reference

Is there a way to automatically load environment variables when activating venv?

I am using python venv to create virtual environments. But, since I am working with several projects with different virtual environments, I don't want to manualy set environment variables every time I switch to a different project.
Is there a way to set venv environment variables automatically when activating the venv?
What is the best practice for this problem?

A good practice is to use dotenv. You can load your environment by placing your environment variables into a file named .env, and whenever you would like to load an environment, just use the lines:
from dotenv import load_dotenv
load_dotenv()
This has the nicety that it only exists within the scope of you running a single script, since it essentially works like calling os.environ['variable'] = 'value' a number of times.

Activating a virtual environment is nothing more than sourcing a shell script. You can edit that script to set whatever variables you like. You will probably also want to edit the definition of deactivate to clear or roll back whatever changes you made to the environment.

you need to write a bash scirpt (in case you are using bash shell), where you specified a particular command which will activate the project python environment and add the project specific envrionment variable in the system environment. and remove the environment variable when you exit the project python environment.
but i don't this is good/correct way to do things. #mz solution will be correct, where you define a .env file and define env variable in it. and use load_env to read the env variable when project runs

This concept is based on Two Scoops of Django. I have implemented it using venv.
Open the Windows PowerShell Script in your virtual environment generated by venv.
The script is located at venv/Scripts/Activate.ps1
At the bottom of the file, you will see this line of code:
$env:VIRTUAL_ENV = $VenvDir
Below that code, enter your environment variable as follow:
$env:VARIABLE_NAME = 'variable_value'
Same concept goes if you are using the Command Prompt to activate the environment, you will need to place environment variables in venv/Scripts/activate.bat

How to source additional environment in pycharm?

I have a ROS application which has a work space with a setup.bash file and another python script with its own virtual environment.
So far this is what I do in my terminal:
1_ pipenv shell (to activate my python virtual environment).
2_ source ../ros_workspace/devel/setup.bash
3_ python some_python_script.py
This code works as I expect.
However, I want to do the same and run this script in pycharm, where my virtual environment is already activated. But how do I source the setup bash additionaly?
My setup.bash file also looks like the following:
What I have tried also is making a "before launch" as follows:

If you set your virtual environment as your interpreter of choice in PyCharm, it will use that particular virtual environment to run its scripts. However, you can also take advantage of some of the functionality that our run configurations provide.
You can check out the "Before Launch" part of the whole configuration window to enter scripts that you want executed.
Once you've set your configurations, you can then go on to run or debug the configuration. Furthermore, if it is just environment variables that you want to source, you can just put in the environment variables in the "Environment Variables" box.
In case you want to run a shellscript, you will need to create a new shell configuration like so:
Once you've added that configuration, you can then go on to reference it later.
You will now see that you can reference that configuration in question:

Python path on windows

I have two installations of Python on a corporate Windows computer. One from the Anaconda distribution:
C:\Users\Me\AppData\Local\Continuum\anaconda3\python3.exe
Another one from a corporate installer:
C:\Users\Me\AppData\Local\Downloaded Apps\WinPython\python-3.4.3\python.exe
If I type "python" in the default "cmd" terminal or in the Git Bash, it says "command not found". Probably for the same reason, Jupyter does not allow me to create a Python 3 notebook. How do I set the paths so that Python is available system-wide? I prefer the Anaconda distribution's Python.
EDIT: Creating a new user variable PYTHONPATH and setting it to C:\Users\Me\AppData\Local\Continuum\anaconda3\ via Windows System Properties does not have an effect.

This answer describes step-by-step an approach that worked for me. However, as eryksun notes in the comment, the additional variable should not be named PYTHONPATH. I renamed it PYTHON, which works.
Strangely, adding the paths directly to the PATH variable did NOT work.

You have to add the path of your installation to the Environment variables.
Simply go to the System Properties / Environment Variables /
From there, create a new system variables and add your python path.

In the system variable section select the Path variable.
Add new variable C:\Users\Me\AppData\Local\Continuum\anaconda3\ as environment variable in your advanced System Settings.
This is from where your system will invoke the python interpreter.
For more details see this answer

How to use environment variables in blender

I added a enviroment variable writing in the ~/.bashrc file this two line
var="stuff.."
export var
using the python interpreter in a normal terminal this two lines of code works
import os
print(os.environ['var'])
but in a blender python console it generate a KeyError so printing os.environ list i can see that there isn't a item with 'var' as key
So i think that is a problem with the environment settings in unix system.
Can anyone help me and explain how export the environment variables for the other processes? Thanks and sorry for the english

The .bachrc file (and similar such as .cshrc) is read when your shell is started, similarly when you start a GUI desktop the shell rc files are read at the time of it starting and the variables at that time are then part of the environment passed onto any GUI apps, changes made while running do not get read in as you start a new app. You can find ways of setting environment variables for different desktops.
One way of passing environment variables into blender is to start it from a terminal window. The rc files will be read when you open the terminal, you can also manually set environment variables before starting blender.
Another way to set environment variables for blender is to start it from a script, this may be one called myblender that will be found in your $PATH or it can also be named blender if it will be found before the real blender. In this script you can set variables before starting blender and any changes will be in effect when you run it.
#!/bin/bash
var="stuff.."
export var
exec /usr/local/bin/blender "$#"

After updating ~/.bashrc you either have to source ~/.bashrc in the terminal where you launch blender or log out and log back in to your system, where the variable should then be in the environment.
If you need to get environment variables that may or may not be available, you can also do something like os.getenv('var', 'default value')

Windows environment variables change when opening command line?

Sometimes when I change my environment variables in Windows, and then use software the depends on those variables, they are not properly updated.
And good example is to change a variable, then open up Windows Command Line and echo the variable and see that it hasn't been changed, even though you properly changed it in the Environment Variables window.
Another example I'm dealing with right now:
I've been using Python 2.4.x for a while for a project, which uses the env var PYTHONPATH who's value has been:
C:\Python24;C:\Python24\lib
Today I installed Python 2.5.x for the project. I changed my PYTHONPATH to be:
C:\Python25;C:\Python25\lib
When I use Python 2.5 to run a script and do this:
import sys
print sys.path
It prints:
'C:\\PYTHON24', 'C:\\PYTHON24\\lib' (and some other Python 2.5 related default installation paths)
So clearly, the old PYTHONPATH environment variable changes aren't really sticking....
Does anyone know why this happens and how to fix it?

When you change an environment variable in the System Properties tab, the new value will propagate to the Windows Explorer, and any apps (such as cmd.exe) opened from the Windows Explorer (or the Run box, Start Menu, etc.) should see the new value.
However, if you're running a program such as an editor or python or some non-Microsoft program launcher, then change an environment variable, and then launch cmd.exe from that program (instead of Windows Explorer) you are likely to see the old value of the environment variable. The reason is that the running program ignored the notification from Windows saying that the environment has changed (not at all unusual), and since the launched process inherits the environment variables, the child process won't see the changes.
The workaround is to make sure you start your app from Windows Explorer or the Run box. Rebooting your machine will work also (if rebooting doesn't solve the problem, then something else is going on).