[issue43899] separate builtin function

[Ammar Askar]

Ammar Askar  added the comment:

Note that the `more_itertools` package on pypi also has a partition method.

Shall we close this off given that a recipe is already present and this is 
available as part of a popular 3rd-party library?

--
nosy: +ammar2

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[issue43899] separate builtin function

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

The drawback of that recipe of partition() is that it calls predicate twice on 
every element. The following implementation calls it only once for every 
element:

def partition(pred, iterable):
t1, t2 = tee((bool(pred(x)), x) for x in iterable)
return (x for p, x in t1 if not p), (x for p, x in t1 if p)

but it may be less efficient for fast predicates.

--
nosy: +serhiy.storchaka

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[issue43899] separate builtin function

[Raymond Hettinger]

Raymond Hettinger  added the comment:

FWIW, here's a recipe from the itertools docs:

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)


Also, here's a more general solution that can handle multiple categories:

>>> from collections import defaultdict
>>> def categorize(func, iterable):
d = defaultdict(list)
for x in iterable:
d[func(x)].append(x)
return dict(d)

>>> categorize(is_positive, [-3, -2, -1, 0, 1, 2, 3])
{False: [-3, -2, -1, 0], True: [1, 2, 3]}
>>> categorize(lambda x: x%3, [-3, -2, -1, 0, 1, 2, 3])
{0: [-3, 0, 3], 1: [-2, 1], 2: [-1, 2]}


At one point, Peter Norvig suggested adding a groupby classmethod to 
dictionaries.  I would support that idea.   Something like:

dict.groupby(attrgetter('country'), conferences)

--
nosy: +rhettinger

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[issue43899] separate builtin function

[David Alvarez Lombardi]

New submission from David Alvarez Lombardi :

I frequently find myself doing the following for lists, sets, and dicts.


passes = [x for x in seq if cond(x)]
fails = [x for x in seq if not cond(x)]


The proposed function would behave similarly to `filter`, but it would return a 
tuple of the passes iterable and fails iterable rather than just the passes.


my_list = [-3, -2, -1, 0, 1, 2, 3]
def is_positive(n):
return n > 0

positives, negatives = separate(function=is_positive, iterable=my_list)


--
messages: 391479
nosy: alvarezdqal
priority: normal
severity: normal
status: open
title: separate builtin function
type: enhancement
versions: Python 3.10

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