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Title: copying WeakValueDictionary is not iteration safe
Type: behavior Stage:
Components: Library (Lib) Versions: Python 3.10, Python 3.9, Python 3.8
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: christian.heimes, djromberg, tpaetz
Priority: normal Keywords:

Created on 2021-01-27 14:42 by djromberg, last changed 2022-04-11 14:59 by admin.

Messages (3)
msg385779 - (view) Author: Daniel Romberg (djromberg) Date: 2021-01-27 14:42
The copy operation is not safe to use during iteration. The following test case raises a "RuntimeError: dictionary changed size during iteration":

import weakref

class Class:

def TEST_weakValue():
  d = weakref.WeakValueDictionary()
  a = Class()
  b = Class()
  d["a"] = a
  d["b"] = b
  e = d.copy()
  for key in e:
    a = None
    c = e.copy()


This is related to where I commented as well, but I couldn't find a way to reopen this issue, which is why I open this one.

We experience a lot fewer crashes in weakref than before had been fixed, however, there are recursive situations in which copy() is invoked while iterating the WeakValueDictionary (e.g., in our case it is a signal/slot implementation where the slots are stored in a WeakValueDictionary). _commit_removals(), which is called at the beginning of the copy operation, might change the dictionary if there are items that are to be removed. If there is an ongoing iteration, the corresponding RuntimeError is raised.

I haven't thought that through entirely, but I wonder whether the copy (and also deepcopy) operation could just blindly copy everything without "committing removals". After the copy, both instances would do their _commit_removals on their own upon access.
msg385783 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2021-01-27 15:40
Do you see a crash or an exception? We use the term "crash" to refer to a segfault or other fatal crashes of the interpreter.
msg385785 - (view) Author: Daniel Romberg (djromberg) Date: 2021-01-27 15:44
Sorry, I meant an exception. I changed the type to "behavior", thanks for the hint.
Date User Action Args
2022-04-11 14:59:40adminsetgithub: 87207
2021-01-27 15:44:33djrombergsettype: crash -> behavior
messages: + msg385785
2021-01-27 15:40:26christian.heimessetnosy: + christian.heimes

messages: + msg385783
versions: - Python 3.6, Python 3.7
2021-01-27 15:37:29tpaetzsetnosy: + tpaetz
2021-01-27 14:42:22djrombergcreate