While this doesn't explain why filter accepts None as the lambda, I think the fact that there are no builtin identity function kind of explains why. It's easier to write None rather than put an identity lambda. I reckon it also makes it easier for a compiler to optimize this specific case.
Thanks for the explanation. I take your point, but in the `filter` case `bool` seems just as easy as `None`, but with the added benefit that no awareness of a special case is needed -- any python programmer should understand how the `bool` builtin works. Curious if there's any downside.
