Re: [matplotlib-devel] Circular references in Figure

[Eric F. <ef...@ha...>]

Michiel de Hoon wrote:
> Hi everybody,
> 
> I was looking at this bug about a memory leak:
> 
> https://sourceforge.net/tracker/?func=detail&atid=560720&aid=2889570&group_id=80706
> 
> While this bug is now essentially fixed, I noticed that the Figure class contains some circular references that prevent it from being cleaned up by the garbage collector, which is effectively a memory leak. At least, for this code:
> 
>>>> from matplotlib.figure import Figure
>>>> fig = Figure()
>>>> del fig
> 
> the garbage collector reports 76 unreachable objects. This seems to be independent of which backend is being used.
> 
> Is it worthwhile to look deeper into it? Or should we accept that it is unavoidable that a library of the size of matplotlib will have circular references?
> 

The purpose of the garbage collector is to handle the case of circular 
references--and mpl is full of circular references.  When there are no 
circular references, then reference counting suffices.  The garbage 
collector fails to handle circular references when an object has a 
__del__ method.  I don't think we have that problem.

I am not actually seeing the result you describe, though.  Using python 
from the interactive prompt--no ipython--when I do gc.collect() and then 
gc.get_count() I get back to the count from before instantiating the Figure.

 >>> gc.get_count()
(4, 0, 0)
 >>> gc.get_count()
(5, 0, 0)
 >>> gc.get_count()
(5, 0, 0)
 >>> gc.get_count()
(5, 0, 0)
 >>> fig = Figure()
 >>> gc.get_count()
(128, 0, 0)
 >>> del(fig)
 >>> gc.get_count()
(128, 0, 0)
 >>> gc.collect()
76
 >>> gc.get_count()
(4, 0, 0)
 >>> gc.get_count()
(5, 0, 0)

Eric

> 
> --Michiel.
>