python - os.getenv and os.environ don't see environment variables of my bash shell

I am on ubuntu 13.04, bash, python2.7.4

The interpreter doesn't see variables I set.

Here is an example:

$ echo $A
5
$ python -c 'import os; print os.getenv( "A" )'
None
$ python -c 'import os; print os.environ[ "A" ]'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/UserDict.py", line 23, in __getitem__
    raise KeyError(key)
KeyError: 'A'

But everything works fine with the PATH variable:

$ echo $PATH 
/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
$ python -c 'import os; print os.getenv("PATH")'
/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

And it notices changes in PATH:

$ PATH="/home/alex/tests/:$PATH"
$ echo $PATH 
/home/alex/tests/:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
$ python -c 'import os; print os.getenv("PATH")'
/home/alex/tests/:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

What could be wrong?

PS the problem comes when using $PYTHONPATH:

$ python -c 'import os; print os.getenv("PYTHONPATH")'
None

Aha! the solution is simple!

I was setting variables with plain $ A=5 command; when you use $ export B="foo" everything is fine.

That is because export makes the variable available to sub-processes:

• it creates a variable in the shell
• and exports it into the environment of the shell
• the environment is passed to sub-processes of the shell.

Plain $ A="foo" just creates variables in the shell and doesn't do anything with the environment.

The interpreter called from the shell obtains its environment from the parent -- the shell. So really the variable should be exported into the environment before.

Those variables (parameters in bash terminology) are not environment variables. You want to export them into the environment, using export or declare -x. See the bash documentation on environment.