It may be time to PEP (or re-PEP), if only to clarify what people are actually asking for.
Brian Sabbey's example from message http://mail.python.org/pipermail/python-dev/2005-March/052202.html *seems* reasonably clear, but I don't see how it relates in any way to "for" loops or generators, except as one (but not the only) use case. def add(thunk1, thunk2, other): print thunk1(1,2) + thunk2(3,4) + other with x,y from add(100): value x*y also a,b: # yikes?? value a*b # this is thunk2 To make my own understanding explicit: (1) Calls for "Ruby blocks" or "thunks" are basically calls for placeholders in a function. These placeholders will be filled with code from someplace else, but will execute in the function's own local namespace. (2) A function as a parameter isn't good enough, because the passed-in function can't see bindings in the surrounding larger function. (It still sees the lexical scope it which it was defined.) (3) A function as a parameter isn't good enough, because the passed-in function can't modify bindings in the surrounding larger function. (4) A thunk could be used today be creating a string (rather than a pre-compiled function) and substituting in the thunk's string (rather than the code to which it would compile) before execution, though this would be ugly. (5) A thunk *might* be do-able with exec or eval if a function's locals were still a dictionary. (6) This has nothing whatsoever to do with for loops or generators, except as a common use case. In particular, thunks may be used to lock/unlock or unwind-protect some resource. Generators are not the only place this is needed, but they have made the "what do you mean, __del__ might never get called?" problem even worse, and Ruby often solves these with a Ruby Block. (7) A __leave__ or __exit__ special method really turns into another name for __del__. It might still be useful if it came with semantics of "I explicitly don't care what order cycles are broken; call me as soon as my object is garbage and who cares if it gets resurrected." There have been recipes (from Tim?) for emulating this by using a proxy to the resource, so that no __del__ cycles can form. -jJ _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com