为什么django.db在不同操作系统上效果不同？

1在windows操作系统上，import django.db一切正常。 2在linux（pip 在centos上安装，源码在suselinux上安装），均有问题。

python
Python 2.6 (r26:66714, May  6 2011, 15:10:21) 
[GCC 4.3.4 [gcc-4_3-branch revision 152973]] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import django
>>> import django.db
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib64/python2.6/site-packages/django/db/__init__.py", line 11, in <module>
    if settings.DATABASES and DEFAULT_DB_ALIAS not in settings.DATABASES:
  File "/usr/local/lib64/python2.6/site-packages/django/conf/__init__.py", line 53, in __getattr__
    self._setup(name)
  File "/usr/local/lib64/python2.6/site-packages/django/conf/__init__.py", line 46, in _setup
    % (desc, ENVIRONMENT_VARIABLE))
django.core.exceptions.ImproperlyConfigured: Requested setting DATABASES, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.
环境变量：
suse-rsyslog:~ # echo $PATH
/sbin:/usr/sbin:/usr/local/sbin:/root/bin:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/X11R6/bin:/usr/games:/usr/lib/mit/bin:/usr/lib/mit/sbin:/usr/local/lib64/python2.6/site-packages/django/bin:/usr/local/lib64/python2.6/site-packages/django/db:/usr/local/lib64/python2.6/site-packages/django/utils:/usr/local/lib64/python2.6/site-packages/django/conf:/usr/local/lib64/python2.6/site-packages/django/

大家谁遇到过吗？