Because you've created a circular reference, the memory won't be freed until the garbage collector runs, detects the cycle, and cleans it up. You can trigger that manually:
import gc
gc.collect() # Memory usage will drop once you run this.
The collector will automatically run occasionally, but only if certain conditions related to the number of object allocations/deallocations are met:
gc.set_threshold(threshold0[, threshold1[, threshold2]])
Set the garbage collection thresholds (the collection frequency).
Setting threshold0 to zero disables collection.
The GC classifies objects into three generations depending on how many
collection sweeps they have survived. New objects are placed in the
youngest generation (generation 0). If an object survives a collection
it is moved into the next older generation. Since generation 2 is the
oldest generation, objects in that generation remain there after a
collection. In order to decide when to run, the collector keeps track
of the number object allocations and deallocations since the last
collection. When the number of allocations minus the number of
deallocations exceeds threshold0, collection starts.
So if you continued creating more objects in the interpreter, eventually the garbage collector would kick on by itself. You can make that happen more often by lowering threshold0, or you can just manually call gc.collect when you know you've deleted one of the objects containing a reference cycle.
