python gc and reference cycles.

파이썬 가비지 콜렉터 관련 정리해둔거 (작성자: azki)

PythonGC_azki.pdf

Since Python makes heavy use of malloc() and free(), it needs a strategy to avoid memory leaks as well as the use of freed memory. The chosen method is called reference counting. The principle is simple: every object contains a counter, which is incremented when a reference to the object is stored somewhere, and which is decremented when a reference to it is deleted. When the counter reaches zero, the last reference to the object has been deleted and the object is freed.

>>> import gc

>>> gc.set_debug(gc.DEBUG_LEAK)

>>> class ss:

...     def __del__(self):

...             print 'del:' + self.str

...

>>> gc.collect()

0

>>> gc.garbage

[]

>>> a = ss()

>>> b = ss()

>>> c = ss()

>>> a.str = 'aa'

>>> b.str = 'bb'

>>> c.str = 'cc'

>>> gc.collect()

0

>>> gc.garbage

[]

>>> del c

del:cc

>>> gc.garbage

[]

>>> gc.collect()

0

>>> a.k = b

>>> b.k = a

>>> del a

>>> del b

>>> gc.garbage

[]

>>> gc.collect()

gc: uncollectable <ss instance at 02560238>

gc: uncollectable <ss instance at 025601C0>

gc: uncollectable <dict 02561780>

gc: uncollectable <dict 025618A0>

4

>>> gc.garbage

[<__main__.ss instance at 0x02560238>, <__main__.ss instance at 0x025601C0>, {'k

': <__main__.ss instance at 0x025601C0>, 'str': 'bb'}, {'k': <__main__.ss instan

ce at 0x02560238>, 'str': 'aa'}]

>>> for a in gc.garbage[-2:]: a['k'] = None

...

>>> gc.garbage

[<__main__.ss instance at 0x02560238>, <__main__.ss instance at 0x025601C0>, {'k

': None, 'str': 'bb'}, {'k': None, 'str': 'aa'}]

>>> del gc.garbage[:]

del:aa

del:bb

>>> gc.garbage

[]

>>> gc.collect()

0