[Python-Dev] PEP 572: Write vs Read, Understand and Control Flow

[Steve Holden]

On Wed, Apr 25, 2018 at 4:56 AM, Tim Peters <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>]
>  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.
​
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