(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.