Optionally include pair of last and first element of iterator in `itertools.pairwise`

[Stefan2 (Stefan)]

@Stefan2

An example of an existing itertool that doesn’t work well with a broken iterator:

from itertools import batched

class BrokenIterator:
  def __iter__(self):
    self.i = 0
    return self
  def __next__(self):
    self.i += 1
    if self.i == 3 or self.i >= 9:
      raise StopIteration
    return self.i

for batch in batched(BrokenIterator(), 5):
    print(batch)

Output:

(1, 2)
(4, 5, 6, 7, 8)

But I wouldn’t blame batched for that.

Ok, I just decided that just like human lawbreakers, such lawbreaking iterators should be put in chains. Literally. So that they can’t illegally continue after their stoppage. Like:

from itertools import batched, chain

for batch in batched(chain(BrokenIterator()), 5):
    print(batch)

Then the output is:

(1, 2)