More precisely, syntactic garbage is data that is unreachable due to the reference graph (there is no path to it), which can be determined by many algorithms, as discussed in tracing garbage collection, and only requires analyzing the data, not the code.
Since Python uses garbage collection, it automatically deallocates the memory that was used for the object so that it may be used again: The Bar instance now resides at the memory location 0x54f30; at the same place as where our previous object, the Foo instance, was located.
However, collecting garbage takes time and, if done manually, requires coding overhead.
Semantic garbage cannot be automatically collected in general, and thus causes memory leaks even in garbage-collected languages.
Detecting and eliminating semantic garbage is typically done using a specialized debugging tool called a heap profiler, which allows one to see which objects are live and how they are reachable, enabling one to remove the unintended reference.