Additional functions in fn.iters

[microamp]

Additional functions to be added to fn.iters (influenced by Underscore.js):

• compact: filter out falsey items from iterable
• reject: the opposite of filter
• groupby: saner approach to itertools.groupby
• countby: similar to groupby, but instead of sub-iterator, it yields a count for the number of items in each group

Unit tests added. Tested with Python 2 (2.7) and Python 3 (3.3).

Thanks for reviewing in advance. Any feedback, positive or negative, would be highly appreciated.

Cheers, James

[microamp]

Will update unit tests in order to make it compatible with Python 2.6.

[microamp]

Unit tests updated and build now passed with Python 2.6 as well.

[kachayev]

Actually, compact is not very useful. There is special filter form in Python:

In [3]: filter(None, [1,0,2,"",3,False])
Out[3]: [1, 2, 3]

which means that compact = partial(filter, None).

[kachayev]

reject is ok, implementation is not obvious. In any case, Haskell variant is good example of functional programming:

reject f = filter (not . f)

It will be good point to implement something like this in fn.py to give example of functional approach.

from operator import not_
from functools import partial

compact = partial(filter, None)
def reject(f, it):
    return filter(compose(not_, id if f is None else f), it)

I didn't check this code, just few ideas.

P.S. id should be defined instead of lambda x: x because it is really widespread case in functional programming.

[kachayev]

I didn't catch idea in groupby. For countby you can use composition of collections.Counter with map.

[microamp]

Thanks for the replies and suggestions, much appreciated.

How does this look for reject?

from operator import not_
from fn.func import F
from fn.underscore import shortcut as _

def reject(func, iterable):
    return filter(F(not_) << (_ if func is None else func), iterable)

even = _ % 2 == 0
assert [2, 4, 6, 8, 10] == list(filter(even, range(1, 11)))
assert [1, 3, 5, 7, 9] == list(reject(even, range(1, 11)))

Or, I can do

return filter(F(_ if func is None else func) >> not_, iterable)

Slightly more concise as it doesn't require extra parens.

P.S. I will open an issue prior to a commit request next time. Thanks, learning a lot here.

[kachayev]

I like first variant, you can even try to do

def reject(func, iterable):
    return filter(F(not_) << (func or _), iterable)

It doesn't check for concrete None value, but useful enough.

[microamp]

Nice. I will open a new pull request for reject tomorrow. Please close this one.

[kachayev]

Will you add compact = partial(filter, None)? And update please README file with new functions (iters block).

[microamp]

Okay, I will add compact as well (unless you find it trivial). And I will update README accordingly. Thanks.