why won't slicing lists raise IndexError?

[Steve D'Aprano]

On Tue, 5 Dec 2017 11:31 pm, Rick Johnson wrote:

> Ned Batchelder wrote:
> [...]
>> Your original statement sounded like, "The else clause can
>> never be executed,"
> 
> No. Of course not. Note that i mentioned _pragmatism_. My
> complaint about the else-clause was not that it could
> _never_ be executed, my complaint that was that the else-
> clause (in Terry's example) serves no useful purpose save to
> act as a semantical placeholder for a non-action.

When you find yourself in a hole, stop digging.

You've already been told that there's no indication or reason to believe that
it is a non-action. You've already been given at least one possible action.
It isn't a non-action, it is two distinct actions:

- the action you take when the slice is non-empty;

- the action you take when the slice is empty.



> Heck, it 
> doesn't even log anything! "Practicality beats purity". For
> instance:
> 
>     (1) Would it be practical to make a shopping list that
>     includes only the products you _don't_ want to buy?

Obviously not *only* the products you don't want, but it is very common to
specify constraints, e.g. what you don't want.

    New car. Must not have an internet connected engine.

    Gluten-free bread.

    Unscented toilet paper.

    Ingredients to make fruit salad. No bananas.

Or alternatives:

    Get a slice of pizza for lunch. If they're out of pizza (i.e. the
    slice is empty) get Chinese food instead.

    
>     (2) How about a list of navigation directions that includes
>     tangential info?
>   
>         "After you take the second left turn, you'll see a big
>         windmill -- just ignore that"

Be careful about rhetorical questions. The answer is not always what you think
it must be.

Giving directions that include landmarks is *very* useful. If you don't see
the windmill, you know you've taken the wrong turn. When you do see it, you
know you're on the right track.


>     (3) How about a recipe that includes non-ingredients?

My wife is an expert at making bread-and-no-butter pudding. This should be
obvious, but in case it isn't, the reason it has that name is because it is a
modified recipe based on (can you guess what?) bread-and-butter pudding, a
British staple.

Also:

    Flourless orange cake.

Taken from a recipe for cooking lentils:

    "Don't add vinegar, as it makes the lentils tough."

(Apparently some people add vinegar when cooking pulses. Curious.)

And for those who are well-educated but not wise:

    "The difference between being educated and being smart is, the 
    educated person knows that tomato is a fruit, while the wise
    person knows not to put tomato in fruit salad."

I can think of a couple of people I would need to write "No tomatoes" on any
fruit salad recipe I gave them.



>> which would have been in direct opposition to the point,
>> that slicing outside the limits would produce an empty
>> list, not an exception.
> 
> And again, that is precisely why i raised an objection to
> this else-clause. Because the inclusion of this else-clause
> is a distraction from the point.

Ah, we've entered Ricksville, where demonstrating that slicing can return an
empty list is taken as distraction from the point that slicing can return an
empty list.

No, wait, you've convinced me Rick! Your logic is impeccable! Except for one
teeny tiny flaw in your reasoning -- you have the conclusion backwards. Its
not the *empty* slice (the else clause) that is the distraction. Its the
*non* empty clause. We already know that slicing returns a list of items. The
part we're discussing is what happens when the slice is out of bounds? Does
it return an empty list, or raise an exception?


alist = bigger_list[start:finish]
if not alist:
    print("Look Ma, no exception was raised!!!")


/only half serious


[...]
>     item = seq[n:n+1]
>     if item:
>         process(item)
>     else:
>         do_without_item()
>          
> "do_without_item()" implies a non-action.

The mind boggles how you get by in real life, when you're so busy digging
yourself into deeper and deeper holes. Do you think about things before you
write them?

"I was going to make spaghetti and meat balls for tea tonight, but the store
was out of mince, so I'll have to do without meat balls."

Does that mean I go hungry and don't eat?


>> Perhaps I have a simulation on a conceptually infinite
>> line, but only store the actual points needed.  The "non
>> action" would be to extend the line.
> 
> That logic does not follow! How do you _extend_ an infinite
> line? :-). 

Ned said that the line stores only the points needed. If you've only used
positions between 1 and 10, only values 1 through 10 have been stored. If you
then try to access position 20, the line is extended to position 20.


> Of course, in the realm of maths, a line is by 
> definition infinite.

Not necessarily. Lines can be finite (e.g. a chord) or infinite, and if
infinite, they can be defined as extending towards infinity in both
directions, or in only one (a ray). 

> But whether you're speaking in metaphor or not, there are
> literally infinite ways in which an else-clause can be
> justified. My argument was simply: "do_without_item()" is
> not a legitimate justification.

Simple and wrong.

Do yourself a favour Rick. Stop digging. If you go much deeper, you'll hit
magma.



-- 
Steve