Why can't environmental variables set in python persist?

Question

I was hoping to write a python script to create some appropriate environmental variables by running the script in whatever directory I'll be executing some simulation code, and I've read that I can't write a script to make these env vars persist in the mac os terminal. So two things:

Is this true?

and

It seems like it would be a useful things to do; why isn't it possible in general?

Answer 1

You can't do it from python, but some clever bash tricks can do something similar. The basic reasoning is this: environment variables exist in a per-process memory space. When a new process is created with fork() it inherits its parent's environment variables. When you set an environment variable in your shell (e.g. bash) like this:

export VAR="foo" 

What you're doing is telling bash to set the variable VAR in its process space to "foo". When you run a program, bash uses fork() and then exec() to run the program, so anything you run from bash inherits the bash environment variables.

Now, suppose you want to create a bash command that sets some environment variable DATA with content from a file in your current directory called ".data". First, you need to have a command to get the data out of the file:

cat .data 

That prints the data. Now, we want to create a bash command to set that data in an environment variable:

export DATA=`cat .data` 

That command takes the contents of .data and puts it in the environment variable DATA. Now, if you put that inside an alias command, you have a bash command that sets your environment variable:

alias set-data="export DATA=`cat .data`" 

You can put that alias command inside the .bashrc or .bash_profile files in your home directory to have that command available in any new bash shell you start.

Answer 2

One workaround is to output export commands, and have the parent shell evaluate this..

thescript.py:

import pipes import random r = random.randint(1,100) print("export BLAHBLAH=%s" % (pipes.quote(str(r)))) 

..and the bash alias (the same can be done in most shells.. even tcsh!):

alias setblahblahenv="eval $(python thescript.py)" 

Usage:

$ echo $BLAHBLAH  $ setblahblahenv $ echo $BLAHBLAH 72 

You can output any arbitrary shell code, including multiple commands like:

export BLAHBLAH=23 SECONDENVVAR='something else' && echo 'everything worked' 

Just remember to be careful about escaping any dynamically created output (the pipes.quote module is good for this)