[Python-Dev] Reference cycle on the module dict (globals()) (original) (raw)

Petr Viktorin encukou at gmail.com
Tue Jan 19 05:39:28 EST 2016

On 01/19/2016 10:42 AM, Victor Stinner wrote:

Hi,

While working on my FAT Python optimizer project, I found an annoying bug in my code. When at least one guard is created with a reference to the global namespace (globals(), the module dictionary), objects of the module are no more removed at exit. Example: --- import sys class MessageAtExit: def del(self): print('del called') # display a message at exit, when messageatexit is removed messageatexit = MessageAtExit() # create a reference cycle: # module -> module dict -> Guard -> module dict guard = sys.Guard(globals()) --- (the code is adapted from a test of testgc) Apply attached patch to Python 3.6 to get the sys.Guard object. It's a minimalist object to keep a strong reference to an object. I expected the garbage collector to break such (simple?) reference cycle. The Guard object implements a traverse module, but it is never called. Did I miss something obvious, or is it a known issue of the garbage collector on modules?

The default type flags are for objects that don't store references. Since you're creating a mutable container, you need to set Py_TPFLAGS_HAVE_GC. See https://docs.python.org/3/c-api/gcsupport.html for all the details.

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