Does it have memory leak?

Question

I read objgraph document recently, and I confused about the following code

>>> class MyBigFatObject(object):
...     pass
...
>>> def computate_something(_cache={}):
...     _cache[42] = dict(foo=MyBigFatObject(),
...                       bar=MyBigFatObject())
...     # a very explicit and easy-to-find "leak" but oh well
...     x = MyBigFatObject() # this one doesn't leak

It show that "a very explict and easy-to-find 'leak'". Do this has memory leak? Is it the dict _cache?

its not a leak, its because there's always a reference to `_cache` as long as `computate_something` is in scope, so it will always remain in memory — GP89, Jul 27 '12 at 08:54

score 7 · Accepted Answer · edited May 23 '17 at 11:52

_cache is a keyword argument with a mutable default, which is indeed mutated in the function body. This'll store MyBigFatObjects instances permanently in that mutable keyword argument.

This is not so much a leak as a misunderstanding of the longevity of mutable keyword arguments. The function itself could still call del _cache[42] to delete these objects.

See "Least Astonishment" and the Mutable Default Argument for more information on the subject.

score 1 · Answer 2 · answered Jul 27 '12 at 08:59

As dict is mutable, the changes made in it will affect also the caller, so the reference to two MyBigFatObject will be kept.

You should explicitly delete it (del _cache[42]), o rather make changes in own copy of dictionary or any other mutable object (you can use copy module for that).

Does it have memory leak?

2 Answers2