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To put it another way, IDLE is written for standard Python. A standard Python interpreter has a getattr builtin that works as documented in the library manual. Deleting that or any other builtin makes the interpreter non-standard. So would replacing or wrapping the builtin with a function that does anything other than innocuously augmenting the behavior with something like counting calls or logging them to a file.
Deletion of getattr *does* create problems for CPython (or any interpreter) in that legal Python code using getattr no longer runs. I presume CPython can limp along and execute other code because it directly calls the C function wrapped by the __builtins__.getattr object.