Daniel Lemire publishes a case where Python set and dict operations run in quadratic time

Sources: primary, discussion
Summary: The post gives three lines that reproduce quadratic time in Python set and dictionary operations. A separate measurement in the same post records a plain dictionary lookup slowing from 21.8 to 201.9 nanoseconds per key under cache pressure alone, with no collisions involved.
Why it matters: Container operations a Python service treats as constant time can degrade by an order of magnitude from data layout, which no algorithmic review of the code would show.