[Python-ideas] all(iterable) should be like all(iterable, key=func)

[Steven D'Aprano]

On Sun, Aug 23, 2015 at 12:01:57PM +0530, shiva prasanth wrote:
> functionality of all(iter) is to check all are true by converting it to
> boolean type
> 
> if it is changed to all(iterable,key=lambda a:bool(a)) it still works and

There is no need for the lambda. key=bool will work the same way, and 
more efficiently.

 
> we can also do  a lot of things like
> all([2,3,4],key=lamdba a:a) gives false

No, it would return True, since all the items are truthy. lambda a: a is 
equivalent to not transforming the items at all, which makes it 
equivalent to using bool as the implied key.

You can test that yourself by using:

f = lambda a: a
all(f(a) for a in [2, 3, 4])


There is no need for a key function, since we can get the same result 
using either map() or a generator expression:

all(map(keyfunction, iterable))

all(keyfunction(obj) for obj in iterable)

whichever you prefer.


There's no obvious one-liner to check whether an iterable contains only 
the same object, but a helper function is easy to write:

def same(iterable):
    it = iter(iterable)
    try:
        first = next(it)
    except StopIteration:
        return False
    return all(obj == first for obj in it)


-- 
Steve