Python: Exporting environment variables in subprocess.Popen(..)

I am having an issue with setting an environment variable on a call to subprocess.Popen. The environment variable does not seem to be getting set. Any suggestions on how to properly set environmental variables for a Python commandline call?

My goal is to run a script that uses an environmental variable determined from my Python code:

d = dict(os.environ)
d["TEST_VARIABLE"] = str(1234)
subprocess.Popen('/usr/bin/mybinary', env=d).wait()

but the script reacts as if the variable has never been set

Here is my attempt to test, using Python's interactive interpreter:

d = dict(os.environ)
d["TEST_VARIABLE"] = str(1234)
subprocess.Popen(['/bin/echo', '$TEST_VARIABLE'], env=d).wait() 

and the output is:

"$TEST_VARIABLE"
0

I thought env=d should set the environment for the subprocess, but it apparently does not. Any suggestions on how to correct this issue?

The substitution of environment variables on the command line is done by the shell, not by /bin/echo. So you need to run the command in a shell to get the substitution:

In [22]: subprocess.Popen('/bin/echo $TEST_VARIABLE', shell=True, env=d).wait()
1234
Out[22]: 0

That doesn't mean the environment variable is not set when shell=False, however. Even without shell=True, the executable does see the enviroment variables set by the env parameter. For example, date is affected by the TZ environment variable:

In [23]: subprocess.Popen(["date"], env={'TZ': 'America/New_York'}).wait()
Wed Oct 29 22:05:52 EDT 2014
Out[23]: 0

In [24]: subprocess.Popen(["date"], env={'TZ': 'Asia/Taipei'}).wait()
Thu Oct 30 10:06:05 CST 2014
Out[24]: 0

For Python 3.5 and newer, you can use unpacking generalizations, eg:

env = {
    **os.environ,
    "TEST_VARIABLE": str(1234),
}
subprocess.Popen('/usr/bin/mybinary', env=env).wait()