On Sat, Dec 14, 2019 at 9:46 PM Andrew Barnert <[email protected]> wrote:
> > Of the top of my head, I can’t think of a single non-contrived example
> of using a bare iterator in case that is not specifically doing something
> “special”.
>
> Calling iter on a container is hardly the only way to get an Iterator. You
> also get Iterators from open, map, filter, zip, genexprs, generator
> functions, most itertools functions, etc.
thanks -- I was being pretty brain dead.
But most of those are usually immediately iterated over again.So in common
use, I'd say the fileobject from open() is a biggie -- that's one of the
few where folks will commonly do something other than simply iterate over
the whole thing at once anyway.
In fact, I think files are one of the most common ways people learn about
> iterators. There are certainly a lot of StackOverflow dups asking why `for
> line in file:` gives them no lines when just 10 lines earlier the same file
> had 20 lines.
>
EXACTLY ! here we have it -- the most common case of folks working directly
with a iterator, and they are confused with "for". So first() wouldn't add
any new source of confusion.
Yes, I can see that folks might do somethig like:
my_file - open(something)
the_header = first(my_file)
and then later on be surprised that another call to first() produces a
different result. But I think they'd only be surprsed once :-)
and file objects are a bit special, because they have a whol other API:
my_file.readline()
in fact, In my code, and most code I've seen, that's exactly what's used:
# get the header
header = my_file.readline()
# parse the rest
for line in my_file():
do_something(line)
I could use next() instead of readline() but I never do. Probably because I
learned Python long before files were an iterator. But I don't' see newbies
doing it, either. (of course, I'm not teaching it that way ;-) ). But I'm
not sure it's just that - I think it's because most of us are writing that
kind of code to deal with any foile-like object, not any generic iterator.
As in: a file object happens to be an iterator (so I can put it in a for
loop), rather than a file object is an iterator, that happens to have some
other methods for working with it.
Now that I think about it, that's also the case with sequences -- we think
of the fact that we are working with a list, and it also happens to be an
iterable, so we can put it in a for loop. rather than thinking about
writing general code for any iterator, that might be a sequence.
Which is why I'd use a_list[0], rather than next(iter(a_list)).
but I can't use dict.keys()[0], for instance.
So it would be nice to have a way to do that that was more obvious than:
next(iter(d.keys())
-CHB
--
Christopher Barker, PhD
Python Language Consulting
- Teaching
- Scientific Software Development
- Desktop GUI and Web Development
- wxPython, numpy, scipy, Cython
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