Pytest: Mocking environment variables

import subprocess

def get_python_version():
    proc = subprocess.Popen(['python', '-V'],
        stdout = subprocess.PIPE,
        stderr = subprocess.PIPE,
    )

    out,err = proc.communicate()
    if proc.returncode:
        raise Exception(f"Error exit {proc.returncode}")
    #if err:
    #    raise Exception(f"Error {err}")
    if out:
        return out.decode('utf8') # In Python 3.8.6
    else:
        return err.decode('utf8') # In Python 2.7.18

import app
import os

def test_python():
    out = app.get_python_version()
    assert out == 'Python 3.8.6\n'

def test_in_path(monkeypatch):
    monkeypatch.setenv('PATH', '/usr/bin')
    out = app.get_python_version()
    assert out == 'Python 2.7.18\n'
    print(os.environ['PATH'])
    print()

def test_other():
    print(os.environ['PATH'])
    print()

def test_keep(monkeypatch):
    monkeypatch.setenv('PATH', '/usr/bin' + os.pathsep + os.environ['PATH'])
    print(os.environ['PATH'])