a common idiom i run into is wanting to add/set one or two env vars when running a command via subprocess.  the only thing the API allows currently is inherting the current environment, or specifying the complete environment.  this means a lot of copying & pasting of the pattern:

 env = os.environ.copy()
 env['FOO'] = ...
 env['BAR'] = ...
 subprocess.run(..., env=env, ...)

it would nice if we could simply express this incremental behavior:

 subprocess.run(..., extra_env={'FOO': ..., 'BAR': ...}, ...)

then the subprocess API would take care of copying & merging.

 if extra_env:
   assert env is None
   env = os.environ.copy()
   env.update(extra_env)

this is akin to subprocess.run's capture_output shortcut.

it's unclear to me whether this would be in both subprocess.Popen & subprocess.run, or only subprocess.run.  it seems like subprocess.Popen elides convenience APIs.

dict syntax tools make it fairy easy to compose new dicts from old ones with overrides:

    subprocess.run(..., env={**os.environ, 'FOO': ..., 'BAR', ...}, ...)

Would this be sufficient to avoid the copy/pasting boilerplate?

Caleb's answer, using PEP 584's merge operator:

    newenv = os.environ | {'FOO': ..., 'BAR': ...}
    subprocess.run(..., env=new_env, ...)

Ah, I didn't realize that os.environ and os.environ b aren't dict subclasses. I've added a ticket to update them with the new operators!

personally i still like having the extra_env setting explicitly broken out, but i agree that those operators help ease the majority of the pain.  i hadn't come across them before as they aren't in Python 2.

i wouldn't be upset if people declined the FR considering those options will be available in Python 3.9+.  thx for the tips.

The dict unpacking generalizations that I posted were added in Python 3.5, which is pretty old by now. (But, true, is in Python 3 and not Python 2). This is the PEP: https://www.python.org/dev/peps/pep-0448/

The new syntax that Brandt posted will indeed only be available from 3.9 on.

I think those dict unpacking and merging options are sufficient that adding an extra_env= parameter would merely complicate the already over complex API.

Thanks for the suggestions folks!