As Christian Heimes describes it here, Python uses its own memory allocator for small objects (< 257 bytes). Larger objects are allocated directly with malloc, smaller objects end up in arenas. The code is well documented in obmalloc.c.

• Classes with a del method may create reference cycles. The GC can't break cycles when a del method is involved.

• keeping references to tracebacks, exception objects (except Exception, err) or frames (sys._getframe())

Related information:

• tips from Christian Heimes to find memory leaking code

• difficulties to find a leak in Edgewall Trac

• reducing the footprint of python applications

• CodingProjectIdeas/PythonGarbageCollected