Bug report

Bug description:

Ideally, defaultdict would provide atomic "compute and add only if key is absent" behavior. When __missing__ is called, another thread might insert the key. Or, it can be inserted between the call to the factory function and the PyObject_SetItem() call.

It would help to call PyDict_SetDefaultRef() so that an existing value is not replaced. That still has the issue that the factory function is called when a value already exists for that key but that is arguably a less serious problem. We should not be replacing an existing value.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-142495 fix __missing__ data race condition #142569
• gh-142495: make defaultdict keep existed value when racing with __missing__ #142668
• [3.14] gh-142495: Make defaultdict keep existed value when racing with __missing__ (GH-142668) #142832
• [3.13] gh-142495: Make defaultdict keep existed value when racing with __missing__ (GH-142668) #142858