[New-bugs-announce] [issue39829] __len__ called twice in the list() constructor

[Kim-Adeline Miguel]

New submission from Kim-Adeline Miguel <kimiguel at 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 <report at bugs.python.org>
<https://bugs.python.org/issue39829>
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