code: func IsolateEnvironment(e *environ.Env, keepPythonPath bool) {
  	if e == nil {
  		return
  	}
  
  	// Remove PYTHONPATH if instructed.
  	if !keepPythonPath {
  		e.Remove("PYTHONPATH")
  	}
  
  	// Remove PYTHONHOME from the environment. PYTHONHOME is used to set the
  	// location of standard Python libraries, which we make a point of overriding.
  	//
  	// https://docs.python.org/2/using/cmdline.html#envvar-PYTHONHOME
  	e.Remove("PYTHONHOME")
  
  	// set PYTHONNOUSERSITE, which prevents a user's "site" configuration
  	// from influencing Python startup. The system "site" should already be
  	// ignored b/c we're using the VirtualEnv Python interpreter.
  	e.Set("PYTHONNOUSERSITE", "1")
  }
docs: // IsolateEnvironment mutates e to remove any environmental influence over
  // the Python interpreter.
  //
  // If keepPythonPath is true, PYTHONPATH will not be cleared. This is used
  // by the actual VirtualEnv Python invocation to preserve PYTHONPATH since it is
  // a form of user input.
  //
  // If e is nil, no operation will be performed.