Pin Object Identity in Tests Before Extracting Mutating Functions

Software developers are advised to freeze object identity checks in tests before extracting any single mutating function from a shared module. Value-based assertions alone can miss alias bugs, since a copied list may pass item checks while silently altering the original object's order or structure. The guidance focuses specifically on in-place container mutations, recommending that developers capture object IDs of mutable arguments before and after each function call to detect unexpected identity changes. A reference table categorizes common leaf-function behaviors — such as in-place sorting or nested dict aliasing — and indicates which are safe to extract only when identity contracts remain stable. The proposal includes a draft test harness to track container IDs and mapping key changes, intended to be adapted locally before any code restructuring begins.