Before v3.9, list literals weren't over-allocated, but this behavior regressed starting with v3.9:

Switched to initializing list literals w/ LIST_EXTEND
https://bugs.python.org/issue39320
#17984

Commit where over-allocation of list literals first appeared
https://bugs.python.org/issue38328
#17114
6dd9b64

This changes to the original behavior of not over-allocating lists for list literals, by not over-allocating any 0-length list that is extended. Technically this is a change in behavior, as lists that are initialized to length 0, and then immediately extended, will not be over-allocated on that initial extension. This change in behavior is likely innocuous, and possibly desirable, whereas the long-standing Python behavior of not using over-allocation for list literals is almost certainly desired.

Adds a test_overallocation test for lists to find regressions (assuming the regression doesn't alter the behavior of both the list-literal and list-initialization from a known length).

https://bugs.python.org/issue43574