When Python exits, not all memory is freed due to several reasons primarily related to the nature of memory management and garbage collection within Python, particularly in its default implementation, CPython.
Circular References
One of the main reasons why all memory isn't freed upon Python's exit is due to circular references among objects. Circular references occur when two or more objects reference each other, creating a loop that prevents Python's garbage collector from determining if they can be safely deallocated. This situation can prevent the reference count of the involved objects from reaching zero, which is necessary for their memory to be released[1][8].
Memory Reserved by the C Library
Python, particularly CPython, relies heavily on the underlying C libraries for its operation. Certain portions of memory allocated by these C libraries cannot be deallocated by Python's garbage collection mechanisms. This is because the control over these memory allocations and deallocations is managed outside of Python's scope, typically by the C runtime environment[1][8].
Garbage Collection Mechanisms
Python uses a combination of reference counting and generational garbage collection to manage memory. While reference counting deals with most objects immediately when their reference count drops to zero, it cannot handle circular references effectively. To address this, Python employs generational garbage collection, which can break circular references but may not always run a collection cycle before the program exits[2][6][10].
Embedded Environments and Extensions
In cases where Python is embedded in other applications or when using C extensions, Python's shutdown process may not control the entire memory cleanup process. The host application or the external libraries might manage memory according to their own rules, which can lead to memory not being freed when Python exits[4][5].
