[Python-Dev] PEP 572: Write vs Read, Understand and Control Flow (original) (raw)
Terry Reedy tjreedy at udel.edu
Wed Apr 25 17:17:50 EDT 2018
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On 4/25/2018 6:10 AM, Steve Holden wrote:
On Wed, Apr 25, 2018 at 4:56 AM, Tim Peters <tim.peters at gmail.com_ _<mailto:tim.peters at gmail.com>> wrote:
[Tim] >> Binding expressions are debugger-friendly in that they don't just >> vanish without a trace. It's their purpose to capture the values of >> the expressions they name. Indeed, you may want to add them all over >> the place inside expressions, never intending to use the names, just >> so that you can see otherwise-ephemeral intra-expression results in >> your debugger ;-)
[Steven D'Aprano <steve at pearwood.info <mailto:steve at pearwood.info>>] wrote: > That's a fantastic point and I'm surprised nobody has thought of it > until now (that I've seen). > > Chris, if you're still reading this and aren't yet heartedly sick and > tired of the PEP wink this ought to go in as another motivating point. You know, I thought I was joking when I wrote that - but after I sent it I realized I wasn't ;-) You just don't realise how perspicacious you truly are, Tim! It would actually be quite convenient, and far less error-prone, to add a binding construct inside a complicated expression for purposes of running under a debugger. The alternative is typing the sub-expression(s) of interest by hand at the debugger prompt, or adding print()s, both of which are prone to introducing typos, or changing results radically due to triggering side effects in the code invoked by the duplicated sub-expression(s). Adding a binding construct wouldn't change anything about how the code worked (apart from possibly clobbering a local name). Indeed, in the cases where I currently find myself unwrapping expressions to capture their values in local variables for debugging purposes it would usually be far less intrusive to bind a name to the expression inline, then use the debugger to inspect the value.
I agree that this is a definite plus feature. Being able to tag subexpressions would make visual debuggers that show all local variables as one steps (like IDLE's) even more useful relative to print statements.
-- Terry Jan Reedy
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