今天遇到的情况，我同事在 virtualenv 里执行 gunicorn 运行 Django app，出来的 Django 错误信息却是 /usr/local/ 路径下面那套 Python 里的：
2015-09-18 15:32:47 [11538] [ERROR] Error handling request
Traceback (most recent call last):
File “/usr/local/lib/python2.7/dist-packages/gunicorn/workers/sync.py“, line 131, in handle_request
respiter = self.wsgi(environ, resp.start_response)
File “/usr/local/lib/python2.7/dist-packages/django/core/handlers/wsgi.py“, line 236, in __call__
self.load_middleware()
File “/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py“, line 57, in load_middleware
raise exceptions.ImproperlyConfigured(‘Middleware module “%s” does not define a “%s” class’ % (mw_module, mw_classname))
ImproperlyConfigured: Middleware module “django.contrib.auth.middleware” does not define a “SessionAuthenticationMiddleware” class
尝试 which gunicorn 返回结果是 virtualenv 里的那个 gunicorn；尝试 strace gunicorn 执行，却能正常执行。
我试了试，把 /usr/local/bin/gunicorn 的x权限取消，再让同事执行，就返回 Permission denied 说明执行的不是 Virtualenv 里的 gunicorn。改试 type gunicorn 返回结果：
gunicorn is hashed (/usr/local/bin/gunicorn)
目前尚不清楚是什么原因造成了该命令被 bash 给 hash 了。source virtualenv的 activate 命令的时候 hash 表会被清空的。