itertools.first ?

Note: Extended with Stop Press! section.

Note: Extended later with "Identity with next" section.

Note: Extended later with "Deviant" section.

I wrote something on Rosetta Code and found myself wanting, (again), a version of the any() function that returned the first True elements value rather than the Boolean value True.

The code was calculating the Harshad numbers defined as a positive integer that is divisible by the sum of its digits. I need to show the first 20 and the one greater than 1000.

I came up with the following:

>>> import itertools
>>> def harshad():
    n=1
    while True:
        if n % sum(int(ch) for ch in str(n)) == 0:
            yield n
        n += 1

>>> list(itertools.islice(harshad(), 0, 22))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 18, 20, 21, 24, 27, 30, 36, 40, 42, 45, 48]
>>> for n in harshad():
    if n > 1000:
        print(n)
        break


1002

It does the job, but I wanted a more functional way of finding the 1000'th element.

I tried to use any but was stuck because it only returns True or False. I could make it work :

>>> tmp = []
>>> any((tmp.append(x), ) for x in harshad() if x > 1000)
True
>>> tmp[0]
1002
>>>

But the above is an obscuring hack to get the value of x out of the genexp.

Needed

What I need is a function that I will call first, that works like any but returns the first value that is true (rather than True) on success, otherwise False.

If any can be defined similarly to:

def any(iterable):
    for value in iterable:
        if value:
            return True
    return False

Then function first would be defined as:

def first(iterable):
    for value in iterable:
        if value:
            return value
    return False

And I could then have written:

>>> first(x for x in harshad() if x > 1000)
1002
>>>

Which is so clean I would think its a candidate for adding to the itertools module.

What do you think?

(I'll add this to Reddit and g+ for discussion as well).

Stop Press!

Darn! I have just worked out that I could have written the following:

>>> (x for x in harshad() if x > 1000).__next__()
1002
>>>

It works and is not so bad looking. Not as good as a first() function, but usable.

Identity with next

aceofears on Reddit pointed me in the direction of the default value for the next() function. That got me thinking and I think I have an identity between my first() and the existing next()

first(iterator) ≡ next((item for item in iterator if item),
                       False) 

If the above holds then I know I'd rather write first(), but could live without it {melodramatic sigh}.

Oh, these should hold too:


any(iterator) ≡ next((True for item in iterator if item),
                     False) 

all(iterator) ≡ next((False for item in iterator if not item),
                     True) 

Deviant

There is an identy missing. any() is to first() as all() is to ???
I would call this missing function deviant(). Its action would be to return the first item from an iterator that was false rather than the Boolean False.

So:


deviant(iterator) ≡ next((item for item in iterator if not item),
                         True) 

(I have not worked out a use case for this as yet though).