OK, now for:
On Wed, May 13, 2020 at 7:50 PM Andrew Barnert <abarn...@yahoo.com> wrote:
> Now onto the stuff that probably nobody else cares about:This is your mail
> agent being a pain again. You’re the one who said that, I quoted you saying
> it, and now you’re agreeing with yourself.
>
Of COURSE I agree ith myself :-) -- but I made that mistake partially
because there was a big thread about this a while back -- and I wasn't the
only one that said that.
> > Students often want to know why this doesn’t work:
> with open("file") as f:
> for line in file:
> do_stuff(line)
> for line in file:
> do_other_stuff(line)
>
> … when this works fine:
>
> with open("file") as f:
> lines = file.readlines()
> for line in lines:
> do_stuff(line)
> for line in lines:
> do_other_stuff(line)
>
> This question (or a variation on it) gets asked by novices every few day’s
> on StackOverflow; it’s one of the top common duplicates.
>
> The answer is that files are iterators, while lists are… well, there is no
> word.
yes, there is -- they are "lists" :-) -- but if you want to be more
general, they are Sequences.
But that’s the wrong generalization. Because sets also work the same way,
and they aren’t Sequences. Nor are dict views, or many of the other kinds
of things-that-can-be-iterated-over-and-over-independently.
But file.readline() does not return any of those objects. It returns a
list. If you see this as an opportunity to teach about the iteration
protocol, then sure, you'd want to make that distinction. But I think the
file object is the wrong first example -- it's an oddball, having both the
iteration protocol, AND methods for doing most of teh same things. Most
iterables don't have the equivalent of readlines() or readline() -- and in
this case, I think THAT's the main sorce of confusion, rather than the
iterable vs iterator distinction.
And you sure would want to make sure they "get" that readlines() is
"greedy" -- it's like calling list() on an iterable. But why introduce that
now?
<snip>
>> Which doesn't require me talking about iterators or iterables, or iter()
or next()
> Sure, which is great right up until they ask the same question about why
they can’t iterate twice over a map or zip.
Then THAT's the time to get into the iteration protocol -- map and zip are
much more "clean" examples.
> > You can explain it anyway. In fact, you _have_ to give an explanation
with analogies and examples and so on, and that would be true even if there
were a word for what lists are. But it would be easier to explain if there
were such a word, and if you could link that word to something in the
glossary, and a chapter in the tutorial.
OK -- time for someone to come up with word for "Itererable that isn't an
Iterator" -- I"d start using it :-)
> The word “view” _is_ great for things-like-dict-keys. That’s why I
started off this thread asking for a view instead of an iterator, which I
thought would be immediately clear. Unfortunately, it isn’t, or we wouldn’t
even be having this discussion.
well it's as clear as maybe anything else -- at least after talking about
it a while ...
- CHB
--
Christopher Barker, PhD
Python Language Consulting
- Teaching
- Scientific Software Development
- Desktop GUI and Web Development
- wxPython, numpy, scipy, Cython
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