delete TransformListener object in python to release computational resource

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Hello! Recently I notice that, after I create a TransformListener object in a class in python, e.g.:

self.tf = TransformListener(),

the created tf object gets stuck in the memory, even I try deleting it in the "__del__(self)" method of the class with:

del self.tf

self.tf = None,

it does not work. The TransformListener object is used by a method of this class. The problem is that when I continue to create the object of this class (which is called by another class during operation), the TransformListener objects accumulate and occupy the computational resources. Thus my question is that how I can get rid of it to free the unnecessarily occupied resources? I appreciate any answers and suggestions in advance!