code: def environ(on=os, **kw):
      
      
      originals = list()
      
      for key in kw:
          originals.append((key, on.environ.get(key, None)))
          on.environ[key] = kw[key]
      
      yield
      
      for key, value in originals:
          if not value:
              del on.environ[key]
              continue
          
          on.environ[key] = value
docs: Update one or more environment variables.
      
      Preserves the previous environment variable (if available) and can be
      applied to remote connections that offer an @environ@ attribute using the
      @on@ argument.