Hi Nick, A couple more issues with PEP 558, as I see it. Lack of specification --------------------- There is no specification section in PEP 558. Much of PEP 558 describes the differences between the current behavior and what PEP 558 proposes. These differences are often intermingled with a discussion of the flaws with the current behavior, which makes it difficult to understand what the proposed behavior is. I could not implement PEP 558 from the PEP alone. Cycles and the cost of maintaining the f_locals cache ----------------------------------------------------- PEP 558 proposes that f_locals acts as a proxy mapping to the locals in a frame *and* that the frame caches the proxy. This obviously creates a cycle, which needs to be broken by the cycle GC. Although the cycle only exists if f_locals is actually used, there is still some overhead for *every* call and return, as the f_locals field needs to be initialized on call and checked for non-NULL on return. Cheers, Mark.