[Python-ideas] Allow filter(items)

[David Mertz]

On Mon, Aug 5, 2013 at 10:22 AM, Shane Green <shane at umbrellacode.com> wrote:
> However:
>
> filter(None, items) and
> filter(lambda item: item, items)
> are alternatives, correct?  If so, then was it also correct to say that None
> as the first parameter is the same as the identity function?

Of course.  The above are equivalent, and both are also equivalent to
the only form *I* would ever think of using: filter(bool, items).
Doing a bool(...) is always implied in evaluation--or maybe more
accurately, applying it is idempotent.

I agree that 'None' as a placeholder predicate is strange and
unintuitive.  I would probably be at least +0.5 on deprecating that,
and requiring predicates be actual callables.  However, that change
*would* break some existing code.

> If those things are true, then I still think passing the iterator itself as
> the first argument is actually more clear and accurate than the current
> approach; either that, or an explicit identity function, as None does not
> have a __call__ that acts as an identity function or boolean evaluation so
> it’s use here doesn’t make much sense to me; it’s just the way it’s always
> worked.

If we could go back in time and reverse the order of 'pred' and 'iter'
in the API of filter(), I'd probably support that.  However, every
single Python program that uses filter() now assumes the current
argument order.  The breakage there is far too great to consider now.


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