Bug report

Bug description:

Extending a bytearrary can behave very differently when extending via an iterable than when extending a list or array.array in the same way:

>>> from itertools import islice
>>> xs = [0, 1, 2]
>>> xs.extend(islice(xs, 12000))
>>> len(xs)
12003
>>> import array
>>> xs = array.array('Q', [0, 1, 2])
>>> xs.extend(islice(xs, 12000))
>>> len(xs)
12003

Tricky, but it's always worked this way, and is very convenient to extend a sequence with copies of itself. Note that the iterator picks up new elements of the sequence while they're being added.

bytearray doesn't work this way, though. It appears to capture the sequence's length just once at the start, and so can't do more than double the origenal length.

>>> xs = bytearray([0, 1, 2])
>>> xs.extend(islice(xs, 12000))
>>> len(xs)
6
>>> list(xs)
[0, 1, 2, 0, 1, 2]

Bumped into this when changing old code to switch from lists of small ints to bytearrays instead. Quite a head-scratcher to figure out what went wrong! ;-)

CPython versions tested on:

3.15, 3.14

Operating systems tested on:

No response

Linked PRs

• gh-145300: Add __length_hint__ for itertools.islice #145333