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(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.