Using ENV vars while running command using subprocess in python 3?

Question

What I am trying to achieve is - to run and return output of some automation scripts, but I am failing to understand why I can't use ENV vars in my scripts?

A simple example I am struggling to implement

import os
import subprocess as sp

cmd = [ 'echo', '$_TEST' ]
myenv = {**{'_TEST':"is it working?"}, **os.environ }
pipe  = sp.Popen( cmd, stdout=sp.PIPE, stderr=sp.PIPE, env=myenv  );
print( pipe.communicate() )

What I am doing wrong?

Update: after getting correct answer.

import os
import subprocess as sp

cmd = [ '/bin/echo', '$_TEST' ]
myenv = {**{'_TEST':"is it working?"}, **os.environ }
pipe  = sp.Popen( " ".join(cmd), stdout=sp.PIPE, shell=True, stderr=sp.PIPE, env=myenv  );
print( pipe.communicate()  )

1. Need to provide a shell argument shell=True (or provide a path to other shell), shell we need to evaluate our variables.

2. list of commands should be replaced by string (eg '/bin/echo $_TEST')

Answer 1

The substitution of environment variables is performed by shell, so your solution just prints original text. The proper way to solve this task:

import os
import subprocess as sp

cmd = 'echo $_TEST'
myenv = {**{'_TEST':"is it working?"}, **os.environ }
pipe  = sp.Popen( cmd, shell=True, stdout=sp.PIPE, stderr=sp.PIPE, env=myenv  );
print( pipe.communicate() )