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Comment by mattlondon | Hacker News Reader

I personally find list comprehensions in python pretty horrible.

They seem to exist only to do lots of stuff in one single line of code. You end up with totally impenetrable unreadable perl-esq garbage write-once-read-never code that is too clever for its own good. And people say python is easy to learn and good for beginners...!

A better approach would be something like Java Streams/.net Lync/RxX pattern IMO. Explicit, clear, no magic, logical.

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You can decompose any list comprehension into it's equivalent for loop fairly easily. I don't see what is so magic about them other than the language having a special compiler level optimization for them?

g = [i for i in list if x == 3] -> g = []; for i in list: if x == 3: g.append(i);

I see people frequently say that list comprehensions are special-cased and highly optimized by the compiler, but I did a few experiments recently with `timeit` and found that using `map()` is on the order of 1.5x faster in all the cases I tested. I think that either winds up being faster than a manual for loop, though

I personally find them extremely readable, and a lot clearer than for loops about the lack of side effects.

I disagree. It’s basically set-builder notation, which probably predates any programming language. It’s a concise way to express filtering.

dic = {k: v for k, v in dic.items() if k in other_dic and v == "bar"}

How could that be improved? That's 3-4 LOC minimum in any other language

My main grip is python's ternary operators, since the True value is evaluated before the condition, if you are doing ternaries on things that might throw exceptions the False value has to come first

value = 0 if key not in dic else dic[key] * 5

rather than (throws indexerror if key isn't in dic)

value = dic[key] * 5 if key in dic else 0

    dic = {k: v for k, v in dic.items() if k in other_dic and v == "bar"}

> "How could that be improved? That's 3-4 LOC minimum in any other language"

If I'm understanding the comprehension correctly,

    (into {} (filter (fn [[k v]] (and (get other-dic k) (= v "bar"))) dic))

Though, for readability, I'd likely write it as:

  (->> dic
       (filter (fn [[k v]]
                 (and (get other-dic k)
                      (= v "bar"))))
       (into {}))

Legibility is in the eye of the beholder.

Each to their own, but both the snippets you posted crossed my threshold for headache inducing parentheses tracking

Similarly, my parsing of special-case syntax in the Python.

What special-case syntax? The only brazenly pythonic thing in there was the (iterative) tuple unpacking, as in

(for) k, v in dic.items()

which is just

k, v = (<key>, <value>) for every key value pair in the dictionary

There are a number of precedence rules you need to keep track of to parse the list comprehension. There are two different syntaxes that do the same thing. Arguably you need to do the same if you don't already know how the threading macro in the my second example works.

I posted due to your claim regarding all other languages necessitating increased verbosity. I should have left it lie, as I didn't intend to promote a language war, just to post a counter example. My apologies.

No language war intended, just curious how something I see as a wheel could be improved.

It appears textually first but the evaluation order is the same as the classic ?: ternary. ```[][0] if False else 'foo'``` will not raise an IndexError.

I like comprehensions, but think this style has some advantages (and some disadvantages):

dic = dic.where((k, v) => k in other_dic and v == "bar").todict()

> dic = {k: v for k, v in dic.items() if k in other_dic and v == "bar"}

dic.iter().filter(|k, v| other_dic.contains(k) && v == "bar").collect();

it's one line, though I'd format it as 3 for readability (list comprehension is hard to read and functional style composes better.

" impenetrable unreadable perl-esq garbage write-once-read-never code " - I had about the same impression when I first saw it. I think ternary operator is as far as I am willing to go ;)

list comprehensions can be elegant and actually improve readability. However, I do agree that it can be easily misused. I have seen many junior Python programmers writing super long, complicated comprehensions that hurt my brain. They think it is pretty cool just because their solutions are one-liners.

List comprehensions are one of those abuse subject features (like most things) where light simple use is fantastic but some people just take it too far into unreadable nastiness.

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