What leaps out at me is that these 3 functions are all straight out of relational algebra style worldview.
Python the language doesn't support relational algebra as a first class concept. The reason it feels like IKEA self assembly is probably because you are implicitly implementing a data model that isn't how Python thinks about collections.
groupBy(fn): {fn(x): x for x in foo}
sortBy(fn): sorted(foo, key=fn)
countBy(fn): Counter(fn(x) for x in foo)
flatten: [x for y in foo for x in y]
filter(fn): [x for x in foo if fn(x)]
For compound operations (e.g. merge two lists, filter and flatten), I find the code to be a lot more easier to "feel" than if you'd combined several functional style functions, where you have to read the function names instead of just seeing the structure.
For example if
“def fn(i): return i//2”
You probably want to get an output like {0: [0,1]} but you’ll only get a scalar for the value (1) and it will be the last one evaluated.
It would be nice if there was a way to do this on one line but you’ll have to do
ret=collections.defaultdict(list) For i In foo: ret[fn(i)].append(i) Return ret
I suppose it isn’t that bad as it will be in the groupBy method.
Or you could add it to a method on a list. I think that’s easier to do with Rust.
g = {fn(x): [y for y in foo if f(y)==f(x)] for x in foo}
It's not the boilerplate that's the problem, it's that using the same construct for everything doesn't convey its intention.
If I see a chain of filter, map, group etc., I instantly know what each part does. I know that filter does a filtering, and then I can look at the function passed to it do know how exactly it's filtering. If I see a list comprehension, I have to first understand if it's filtering or mapping or whatever (or even all at once), before I can grok it.
filter(fn): [x for x in foo if fn(x)]
> About 12 years ago, Python aquired lambda, reduce(), filter() and map(), courtesy of (I believe) a Lisp hacker who missed them and submitted working patches. But, despite of the PR value, I think these features should be cut from Python 3000.
("Python 3000" was the code name for Python 3.) He eventually stepped back from this hard line view but it remains a fact that it's a bit of a historical accident that it's present at all.
[1] https://www.artima.com/weblogs/viewpost.jsp?thread=98196
Most popular languages these days have aligned patterns for common collections operations. Python stands out IMO.
I wrote a bit more details in a parent comment (https://news.ycombinator.com/item?id=27782030)
I'm working on some python these days and I find quite unpleasant the 1-line lambdas, and general messy options to implement common collections operations.