Idiomatically negate a filter

Question

What's the most idiomatic way of writing a filter with a negation?

Example:

is_even= lambda x : x % 2 == 0
odd_numbers= filter( lambda x: not is_even(x), range(10) )

Of course, you can just use list comprehensions - but then you needn't use filter anyway

In case anyone wonders, I've stumbled upon this while trying to split a list based on a condition

Catiline

In case anyone wonders, I'm trying to split a list based on a condition – Catiline

Is that missin ) a typo? – Midgut

Damn, you edited it 1s after I posted. That's too fast :P .. – Midgut

Can you tell us what you're actually trying to do? A broader description please. What's wrong with what you posted? – Sloane

@ReutSharabani I mentioned that in the comments, but I'll move it to the question itself. There's nothing wrong with what I wrote, I was just wondering if there's a more idiomatic or less verbose way of doing it – Catiline

There is no need to write two lambda functions. How about odd = filter(lambda x: x % 2, range(10))? – Transference

The itertools module has ifilterfalse(), which filters for elements where the function returns False. (itertools.filterfalse() in Python 3). No built-in equivalent, but not seems simple enough. – Organology

@HansThen For the sake of the example, assume is_even is already defined – Catiline

@Organology ifilterfalse seems exactly what I was looking for, and exists for python 2. If you care to submit an answer, I'll accept it – Catiline

Great. I tried to make the difference between Python 2 and 3 clear in the answer. Your example (in the question) actually gives different results in 2 and 3. – Organology

Answer 1

The itertools module includes both ifilter() and ifilterfalse(), which filter elements where the function returns True and False respectively.

odd_numbers = ifilterfalse(is_even, range(10))

Note that, in Python 2, there is a difference between filter and ifilter: odd_numbers here would be an iterator, while filter() would give a list (see itertools.ifilter Vs. filter Vs. list comprehensions). If you actually want to build a list, your example with not seems fine, assuming you are set on using filter - list comprehensions may be more 'idiomatic' (List filtering: list comprehension vs. lambda + filter).

In Python 3, filter() constructs an iterator, not a list, and itertools.filterfalse() is the complement.

Organology

Does anyone have any insight into why the authors chose ifilterfalse instead of ireject? – Silvana

Answer 2

Splitting based on a predicate is called a partition. I would find it more idiomatic to implement partition as a separate function rather than repeat its internals specifically for odd and even numbers. Python 3's Itertools Recipes has the following implementation:

def partition(pred, iterable):
    'Use a predicate to partition entries into false entries and true entries'
    # partition(is_odd, range(10)) --> 0 2 4 6 8   and  1 3 5 7 9
    t1, t2 = tee(iterable)
    return filterfalse(pred, t1), filter(pred, t2)

It uses filterfalse (as described by @Lack) and tee defined in that module. So your highest-level code would look like:

odds, evens = partition(is_even, range(10))

Midshipmite

Yes, this is a better solution to the actual problem if you are using both parts of the list. – Organology

This is probably worth posting in the question I linked to, as nobody mentioned it yet. My question in particular was more motivated by curiosity than the practical need to solve the actual partitioning (for which there's a lot of answers already) – Catiline