Some common situations that may prevent the reference count of an object from going to zero include: circular references between objects (e.g., a doubly-linked list or a tree data structure with parent and child pointers); a reference to the object on the stack frame of a function that caught an exception (the traceback stored in sys.exc_traceback keeps the stack frame alive); or a reference to the object on the stack frame that raised an unhandled exception in interactive mode (the traceback stored in sys.last_traceback keeps the stack frame alive).

import gc
import sysclass CGcLeak(object):def __init__(self):self._text = '#' * 10def __del__(self):passdef make_circle_ref():_gcleak = CGcLeak()print "_gcleak ref count0: %d" %(sys.getrefcount(_gcleak))del _gcleaktry:print "_gcleak ref count1 :%d" %(sys.getrefcount(_gcleak))except UnboundLocalError:           # 本地变量xxx引用前没定义print "_gcleak is invalid!"
def test_gcleak():gc.enable()                         #设置垃圾回收器调试标志gc.set_debug(gc.DEBUG_COLLECTABLE | gc.DEBUG_UNCOLLECTABLE | gc.DEBUG_INSTANCES | gc.DEBUG_OBJECTS)print "begin leak test..."make_circle_ref()print "\nbegin collect..."_unreachable = gc.collect()print "unreachable object num:%d" %(_unreachable)print "garbage object num:%d" %(len(gc.garbage))   #gc.garbage是一个list对象，列表项是垃圾收集器发现的不可达（即垃圾对象）、但又不能释放(不可回收)的对象，通常gc.garbage中的对象是引用对象还中的对象。因Python不知用什么顺序来调用对象的__del__函数，导致对象始终存活在gc.garbage中，造成内存泄露 if __name__ == "__main__": test_gcleak()。如果知道一个安全次序，那么就可以打破引用焕，再执行del gc.garbage[:]从而清空垃圾对象列表
if __name__ == "__main__":test_gcleak()

begin leak test...
_gcleak ref count0: 2         #对象_gcleak的引用计数为2
_gcleak is invalid!           #因为执行了del函数，_gcleak变为了不可达的对象begin collect...              #开始垃圾回收
unreachable object num:0      #本次垃圾回收发现的不可达的对象个数为0
garbage object num:0          #整个解释器中垃圾对象的个数为0

import gc
import sysclass CGcLeak(object):def __init__(self):self._text = '#' * 10def __del__(self):passdef make_circle_ref():_gcleak = CGcLeak()_gcleak._self = _gcleak     #自己循环引用自己print "_gcleak ref count0: %d" %(sys.getrefcount(_gcleak))del _gcleaktry:print "_gcleak ref count1 :%d" %(sys.getrefcount(_gcleak))except UnboundLocalError:print "_gcleak is invalid!"def test_gcleak():gc.enable()gc.set_debug(gc.DEBUG_COLLECTABLE | gc.DEBUG_UNCOLLECTABLE | gc.DEBUG_INSTANCES | gc.DEBUG_OBJECTS)print "begin leak test..."make_circle_ref()print "\nbegin collect..."_unreachable = gc.collect()print "unreachable object num:%d" %(_unreachable)print "garbage object num:%d" %(len(gc.garbage))if __name__ == "__main__":test_gcleak()

begin leak test...
gc: uncollectable <CGcLeak 00000000026366A0>
_gcleak ref count0: 3
_gcleak is invalid!
gc: uncollectable <dict 0000000002667BD8>begin collect...
unreachable object num:2       #本次回收不可达的对象个数为2
garbage object num:1           #整个解释器中垃圾个数为1

import gc
import sysclass CGcLeakA(object):def __init__(self):self._text = '$' * 10def __del__(self):passclass CGcLeakB(object):def __init__(self):self._text = '$' * 10def __del__(self):passdef make_circle_ref():_a = CGcLeakA()_b = CGcLeakB()_a.s = _b_b.d = _aprint "ref count0:a=%d b=%d" %(sys.getrefcount(_a), sys.getrefcount(_b))del _adel _btry:print "ref count1:a%d" %(sys.getrefcount(_a))except UnboundLocalError:print "_a is invalid!"def test_gcleak():gc.enable()gc.set_debug(gc.DEBUG_COLLECTABLE | gc.DEBUG_UNCOLLECTABLE | gc.DEBUG_INSTANCES | gc.DEBUG_OBJECTS)print "begin leak test..."make_circle_ref()print "\nbegin collect..."_unreachable = gc.collect()print "unreachable object num:%d" %(_unreachable)print "garbage object num:%d" %(len(gc.garbage))if __name__ == "__main__":test_gcleak()

begin leak test...
ref count0:a=3 b=3
_a is invalid!begin collect...
unreachable object num:4
garbage object num:2
gc: uncollectable <CGcLeakA 00000000022766D8>
gc: uncollectable <CGcLeakB 0000000002276710>
gc: uncollectable <dict 00000000022A7E18>
gc: uncollectable <dict 00000000022DF3C8>