support for ignoring inherited environment

[jesteria]

The plumbum LocalMachine default of passing the parent process environment variables to Popen makes sense; however, it is either non-obvious or non-trivial to disable this behavior for command execution without the inherited environment – in a thread-safe manner in particular.

(I've written a good bit below; but, I'm curious if I'm missing anything or if there are any thoughts on how this might be approached in plumbum.)

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Generally, it now appears that this might be done as follows:

with local.env():
    local.env.clear()

    # run commands
    local['env']()

However:

• It is non-obvious that clear() must be called here – (perhaps it's worth documenting)

• It might instead make sense, (though it also might not), that when local.env() is called without arguments that local.env.clear() is called on the user's behalf – (otherwise, why are they calling env() at all?) I.e.:

  if not args and not kwargs:
      self.clear()

• (As a side note: the local.env() context manager constructor accepts *args, **kwargs, and documents both; yet, args are ignored.)

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Matters appear worse for disabling the inherited environment in a thread-safe manner:

• Because the LocalEnv of local.env is a class-level attribute of LocalMachine, a purpose-dedicated machine may be constructed only from a subclass.
• A LocalCommand retrieved from such a subclass of LocalMachine still executes in the environment of plumbum.local – because LocalCommand.machine simply returns plumbum.local (rather than storing the machine from which its was retrieved).
  • And, the LocalMachine construction of LocalCommand (its choice of constructor) is not trivially configurable/changeable.

Certainly, worrying about the above might be avoided by enabling LocalMachine._popen to somehow ignore its own env (local.env); though, it is not obvious how to achieve this in the "nicest" way. (E.g.: Another keyword argument: _popen(…, isolate=True)? A very special env to pass like: _popen(…, env=IsolatedEnv(…))?)

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As a final note, obviously, all of this can be avoided by nesting commands under /usr/bin/env; though, this defeats the purpose of environment management in plumbum, and it is a shame when subprocess.Popen itself supports a cleared environment straight-forwardly.