Python 3.8 will introduce Assignment Expressions. It is a new symbol: := that allows assignment in (among other things) comprehensions. This new operator is also known as the walrus operator. It will introduce a lot of potential savings w.r.t. computation/memory, as can be seen from the following snippet of the above linked PEP (formatting adapted … Read more
How ’bout: some_array.map {|x| x % 2 == 0 ? x * 3 : nil}.compact Slightly cleaner, at least to my taste, and according to a quick benchmark test about 15% faster than your version…
The other respondents were correct in answering that you had discovered a generator expression (which has a notation similar to list comprehensions but without the surrounding square brackets). In general, genexps (as they are affectionately known) are more memory efficient and faster than list comprehensions. HOWEVER, it the case of ”.join(), a list comprehension is … Read more
The current answers are good, but do not talk about how they are just syntactic sugar to some pattern that we are so used to. Let’s start with an example, say we have 10 numbers, and we want a subset of those that are greater than, say, 5. >>> numbers = [12, 34, 1, 4, … Read more
lst = [j + k for j in s1 for k in s2] or lst = [(j, k) for j in s1 for k in s2] if you want tuples. Like in the question, for j… is the outer loop, for k… is the inner loop. Essentially, you can have as many independent ‘for x … Read more
[x for x in (1,2,3) ] works fine, so you can pretty much do as you please. I’d personally prefer [something_that_is_pretty_long for something_that_is_pretty_long in somethings_that_are_pretty_long] The reason why \ isn’t appreciated very much is that it appears at the end of a line, where it either doesn’t stand out or needs extra padding, which has … Read more
It is very anti-Pythonic to do so, and any seasoned Pythonista will give you hell over it. The intermediate list is thrown away after it is created, and it could potentially be very, very large, and therefore expensive to create.
List comprehensions leak the loop control variable in Python 2 but not in Python 3. Here’s Guido van Rossum (creator of Python) explaining the history behind this: We also made another change in Python 3, to improve equivalence between list comprehensions and generator expressions. In Python 2, the list comprehension “leaks” the loop control variable … Read more
Double list comprehension: [f(x) for x in range(5) for f in (f1,f2)] Demo: >>> f1 = lambda x: x >>> f2 = lambda x: 10*x >>> [f(x) for x in range(5) for f in (f1,f2)] [0, 0, 1, 10, 2, 20, 3, 30, 4, 40]
You got the order wrong. The if should be after the for (unless it is in an if-else ternary operator) [y for y in a if y not in b] This would work however: [y if y not in b else other_value for y in a]