[Python-ideas] Retrying EAFP without DRY (original) (raw)

Ron Adam ron3200 at gmail.com
Mon Jan 23 20:06:52 CET 2012

On Mon, 2012-01-23 at 13:34 +0900, Stephen J. Turnbull wrote:

Nick Coghlan writes: > On Mon, Jan 23, 2012 at 7:30 AM, Mike Meyer <mwm at mired.org> wrote: > > or wrapping a loop around the code when it wasn't really a loop. > > You want to do the same thing more than once: that's a loop.

That's a question of point of view. If "thing" is thought of as a "try" (an operation that might fail), yes, he wants to do the same thing a nondeterministic number of times, and in general, more than once: it's a loop. If "thing" is thought of as a "block" (which isn't a try), then the "things" done with and without exception are different: it's not a loop, it's a conditional. Looking at his example "superficially" (I intend no deprecation by that word, just a point of view in looking at the code), I have some sympathy for Mike's claim that "it's not a loop and it violates DRY." I "know what he means" (and I bet you do, too!) However, when I try to define that, even informally, I arrive at the paragraph above, and I end up coming down on the side that you can't consistently claim that "it's not a loop" and claim that "it violates DRY", in some deeper sense.

You also have to consider the size and scope of the block. Anything more than one or two simple instructions will be difficult to achieve in a clean way. In the case of only one or to instruction, a try block works just fine.

For larger scopes, I think it would require some form of "UNDO_BLOCK" opcode, so that the visited frame states can be restored before a retry attempt is done. It would break if any c code functions visits any frame that is outside the frames the byte code is executing in. I'd also be concerned about memory usage and speed, because it could lead to some very inefficient routines.

Cheers, Ron

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