My class defines __del__ but it is not called when I delete the object.
There are several possible reasons for this.
If your data structures contain circular links (e.g. a tree where each child has a parent reference and each parent has a list of children) the reference counts will never go back to zero. Once in a while Python runs an algorithm to detect such cycles, but the garbage collector might run some time after the last reference to your data structure vanishes, so your __del__ method may be called at an inconvenient and random time. This is inconvenient if you’re trying to reproduce a problem. Worse, the order in which object’s __del__ methods are executed is arbitrary. You can run gc.collect to force a collection, but there are pathological cases where objects will never be collected.
Despite the cycle collector, it’s still a good idea to define an explicit close() method on objects to be called whenever you’re done with them. The close() method can then remove attributes that refer to subobjecs. Don’t call __del__ directly — __del__ should call close() and close() should make sure that it can be called more than once for the same object.
Another way to avoid cyclical references is to use the weakref module, which allows you to point to objects without incrementing their reference count. Tree data structures, for instance, should use weak references for their parent and sibling references (if they need them!).
If the object has ever been a local variable in a function that caught an expression in an except clause, chances are that a reference to the object still exists in that function’s stack frame as contained in the stack trace. Normally, calling sys.exc_clear will take care of this by clearing the last recorded exception.