vpython: Forcefully clear PYTHON* environment.

vpython/run.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package vpython
 
 import (
         "os"
         "os/exec"
         "os/signal"
@@ skipping 48 matching lines @@
 
         // Create our virtual environment root directory.
         err := venv.With(c, opts.EnvConfig, opts.WaitForEnv, func(c context.Context, ve *venv.Env) error {
                 // Build the augmented environment variables.
                 e := opts.Environ
                 if e.Len() == 0 {
                         // If no environment was supplied, use the system environment.
                         e = environ.System()
                 }
 
+                // Remove PYTHONPATH and PYTHONHOME from the environment. This prevents them
+                // from being propagated to delegate processes (e.g., "vpython" script calls
+                // Python script, the "vpython" one uses the Interpreter's IsolatedCommand
+                // to isolate the initial run, but the delegate command blindly uses the
+                // environment that it's provided).
+                //
+                // Also 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.Remove("PYTHONPATH")
+                e.Remove("PYTHONHOME")
+                e.Set("PYTHONNOUSERSITE", "1")
+
                 e.Set("VIRTUAL_ENV", ve.Root) // Set by VirtualEnv script.
                 if ve.EnvironmentStampPath != "" {
                         e.Set(EnvironmentStampPathENV, ve.EnvironmentStampPath)
                 }
 
                 // Prefix PATH with the VirtualEnv "bin" directory.
                 prefixPATH(e, ve.BinDir)
 
                 // Run our bootstrapped Python command.
                 cmd := ve.Interpreter().IsolatedCommand(c, opts.Args...)
@@ skipping 68 matching lines @@
         components = append([]string(nil), components...)
 
         // If there is a current PATH (likely), add that to the end.
         cur, _ := env.Get("PATH")
         if len(cur) > 0 {
                 components = append(components, cur)
         }
 
         env.Set("PATH", strings.Join(components, string(os.PathListSeparator)))
 }