[Python-ideas] if-statement in for-loop

[Ken Kundert]

In my experience it is exceptions and inconsistencies that consume 'working
memory in the brain of humans'. By eliminating the distinction between list
comprehensions and for loops we would be making the language simpler by
eliminating an inconsistency.

Furthermore, I do not believe it is valid to discard a potentially good idea
simply because if taken to extreme it might result in ugly code. With that
justification one could reject most ideas. The fact is, that in many cases this
idea would result in cleaner, more compact code. We should be content to offer
a language in which it is possible to express complex ideas cleanly and simply,
and trust our users to use the language appropriately.

For example, it was suggested that one could simplify a multi-level loop by
moving the multiple levels of for loop into a separate function that acts as
generator. And that is a nice idea, but when writing it, the writing of the
generator function represents a speed bump. Whereas writing something like the
following is simple, compact, quick, and obvious. There is no reason why it
should not be allowed even though it might not always be the best approach to
use:

    for i in range(5) for j in range(5) for k in range(5):
        ...

And I would really like to be able to write loops of the form:

    for item in items if item is not None:
       ...

It is something I do all the time, and it would be nice if it did not consume
two levels on indentation.

-Ken

On Tue, Oct 04, 2016 at 01:31:22PM +1000, Nick Coghlan wrote:
> On 4 October 2016 at 08:18, Erik <python at lucidity.plus.com> wrote:
> > The expression suggested could be spelled:
> >
> > for i in range(10): if i != 5:
> >     body
> >
> > So, if a colon followed by another suite is equivalent to the same construct
> > but without the INDENT (and then the corresponding DEDENT unwinds up to the
> > point of the first keyword) then we get something that's pretty much as
> > succinct as Dominik suggested.
> 
> What's the pay-off though? The ultimate problem with deeply nested
> code isn't the amount of vertical whitespace it takes up - it's the
> amount of working memory it requires in the brain of a human trying to
> read it. "This requires a lot of lines and a lot of indentation" is
> just an affordance at time of writing that reminds the code author of
> the future readability problem they're creating for themselves.
> 
> Extracting named chunks solves the underlying readability problem by
> reducing the working memory demand in reading the code (assuming the
> chunks are well named, so the reader can either make a useful guess
> about the purpose of the extracted piece without even looking at its
> documentation, or at least remember what it does after looking it up
> the first time they encounter it).
> 
> By contrast, eliminating the vertical whitespace without actually
> reducing the level of nesting is merely hiding the readability problem
> without actually addressing it.
> 
> Cheers,
> Nick.
> 
> -- 
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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