Hi,
On 4/22/06, Georg Dahn <[EMAIL PROTECTED]> wrote:
> In my opinion one has done something wrong if one wants to get a sublist
> whith wrong bounds. That's why an error has to be produced. Returning an
> empty list looks like everything is ok where it is not. It takes some
> time to find an error if no error message is produced in such cases.
Why is this "wrong"? And wrong is very subjective... I can agree that
getting a non empty slice where none exists is "wrong". Why is mereley
querying for out-of-bounds "wrong"? By your logic, the get() function
for dictionary objects is "wrong". The programmer should be forced to
write an if statement every time he could have done the same thing with
get(). A lot of time, there is an expectation of default values...
Besides, as I elaborated in my previous email (and which you didn't
address) is that returning an empty list is at least from a mathematical
perspective correct. Your statement "returning an empty list looks like
everything is ok" is also completely subjective. As a programmer I know
that getting an empty list implies that my range is out of bounds. Why
should the language get in the way with statements like "you are
definitely doing something wrong".
> > The most common use-case for the slice operation I can think of is:
> >
> > for item in somelist[M:N]
> > " do something
> > endfor
>
> This is a good example. M and N are not further described here. But M
> and N must come from somewhere. If they are out of bounds, then in many
> or even most cases the programmer has done something wrong. Even if
Why?! Why is this "wrong"? Suppose I have a function which asks the user
for a long list and then wants to use not more than the first three
elements. Or for that matter, maybe the first argument needs to be
discarded or used differently... I can easily imagine many cases like
this, where producing an error is needless intrusion and produces more
complicated looking code where the intent was actually simple.
A real world example is in vim-latex, where the user is expected to
create a setting Tex_HotKeyEnvironments. This is a list of environments
which needs to be mapped to the function keys. Since only the first
4 function keys should be mapped, I would have done something like:
for item in Tex_HotKeyEnvironments[:3]
" do something
endfor
Is it an "error" for the user to specify a list less than 4 long?
I think not!
> there may be some cases where this could make sense, it will be better
> to add an if. An error is no intrusion as you call it, but a very
> helpful signal that there is something wrong.
Once again, in many cases, the list not containing elements given in the
range is not an error but an expected use-case. It is in these cases
where producing an error is annoying. You can always use "if len(list)"
statements where you actually care about the lenght of the list.
> > Having a[M:N] never be an error makes the above work flawlessly.
>
> What does 'flawlessly' mean here? Not to get an error is not good if
> there is one. What may result in more harm: an additional if or writing
> a good code, where your examples doesn't exist, or not producing an
I do not thinking that stuffing code with needless if statements makes
"good code".
> But somewhere you have assigned some values to M and N. That will be
> done dynamically, too. If they are out of bounds, a highly probable
> reason for that is, that you have done something wrong.
>
In many many cases, there is no fault in logic. I fully expect that the
list might be shorter and the
for item in list[M:N]
" do something
endif
automatically handles this since the loop is never entered if the range
is out of bounds.
> But these languages have in common, that it is very hard to find some
> types of errors, which can be avoided in more restricting languages.
I personally feel that Python is one of the best languages around as far
as language design goes. There are usually extremely good reasons why
things are done the way they are. In this case, I happen to completely
agree with it. In all the cases where I have used lists, the if
statement will be completely redundant. I am asking for this due to
real-word use-cases, not due to some abstract feeling of "right" or
"wrong".
Best wishes,
Srinath
