用objgraph包来绘制引用关系时遇到的错误
ImportError: No module named objgraph
#pip install xdot
#pip install objgraph
AttributeError: ‘module’ object has no attribute ‘F_GETFD’
Image renderer (dot) not found, not doing anything else
xdot是graphviz（http://www.graphviz.org/）的一个命令，安装graphviz即可
#pip install graphviz
或者
#yum install graphviz
恢复正常
