On 7/23/2020 5:14 AM, Peter Slížik wrote:
Works in what way? You can't use it in a 'for' loop if it doesn't
define __iter__.
class Iterable:
def __iter__(self):
return Iterator(...)
class Iterator:
def __next__(self):
return <next item>
# No __iter__ here.
# I've just forgotten to def it.
Which means that an Iterator is not an iterator.
With this setup, using for to iterate over Iterable *will* still work,
though you cannot use the trick described below.
Yes? It is indeed an iterator, since its iter method returns itself. It
is also iterable, since it has an iter method.
Of course, you're right. But you're reading the article through the eyes of
an experienced developer.
It's so you can do things like this:
def show_with_header(stuff):
stuff = iter(stuff)
hdr = next(stuff)
print(hdr)
for line in stuff:
print(line)
if new_page(): print(hdr)
Yes, @Terry had given the same example. Frankly, I didn't know about it
before, I somehow had the misconception that for always got a 'fresh'
iterator...
The way 'for' gets its iterator is to call iter(iterable). If the
iterable is an iterator, iter(iterator) returns iterator. Open files
are *iterators*.
>>> file.__iter__() is file
True
This means that one can do things like
file = open(...)
for line in file: # Section 1 of file.
if separator(line): break
process1(line)
process_sep(line) # Special line that broke iteration.
for line in file: # Section 2 of file.
process2(line)
One alternative is something like
file = open(...)
section = 1
for line in file:
if section == 1:
if separator(line):
process_sep(line)
section = 2
continue
process1(line)
continue
else:
process2(line)
Another is to use while loops and explicit next(line) calls. The ':='
assignment operator makes this a bit easier. I used to commonly process
files with multiple sections and different 'key' lines marking them.
--
Terry Jan Reedy
--
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