New submission from Kim-Adeline Miguel <kimig...@microsoft.com>:
(See #33234)
Recently we added Python 3.8 to our CI test matrix, and we noticed a possible
backward incompatibility with the list() constructor.
We found that __len__ is getting called twice, while before 3.8 it was only
called once.
Here's an example:
class Foo:
def __iter__(self):
print("iter")
return iter([3, 5, 42, 69])
def __len__(self):
print("len")
return 4
Calling list(Foo()) using Python 3.7 prints:
iter
len
But calling list(Foo()) using Python 3.8 prints:
len
iter
len
It looks like this behaviour was introduced for #33234 with PR GH-9846.
We realize that this was merged a while back, but at least we wanted to make
the team aware of this change in behaviour.
----------
components: Interpreter Core
messages: 363186
nosy: brett.cannon, eric.snow, kimiguel, pablogsal, rhettinger
priority: normal
severity: normal
status: open
title: __len__ called twice in the list() constructor
type: behavior
versions: Python 3.8, Python 3.9
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue39829>
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