Python 嵌套 import 加载文件, 出现问题求解决

from test2 import pig
print 'hello'

from pig import pig

from test1 import dog

class pig(object):
    pass

print 'pig'

from dog import dog
from dog1 import dog1

class dog(object):
    pass

print 'dog'

from test2 import pig

class dog1(object):
    pass

print 'dog1'

Traceback (most recent call last):
dog
  File "/opt/svn/tor/other/main.py", line 2, in <module>
    from test2 import pig
  File "/opt/svn/tor/other/test2/__init__.py", line 2, in <module>
    from pig import pig
  File "/opt/svn/tor/other/test2/pig.py", line 2, in <module>
    from test1 import dog
  File "/opt/svn/tor/other/test1/__init__.py", line 2, in <module>
    from dog1 import dog1
  File "/opt/svn/tor/other/test1/dog1.py", line 1, in <module>
    from test2 import pig
ImportError: cannot import name pig