In the image there is a 3d letter O from dxf file. O represented as two rounds.so there is two entity.
How can i detect one entity is inside another (in case of o,how to know small round is inside big round)?
In this case, calculate the bounding box around both entities.
This has the form of a tuple, say (xmin, xmax, ymin, ymax). You can think of this as a box with sides parallel to the x- and y-axes that surrounds the entity.
If both the x-values from the first bounding box are in between the xmin and xmax of the second bounding box, and the same goes for the y-values, the first bounding box is inside the second one.
Note that this technique is an approximation. It is possible to create entities that overlap while their bounding boxes don't intersect.
