You can use a generator expression to do the filtering, then use islice() to limit the number of iterations: from itertools import islice filtered = (i for i in a if i == 1) b = list(islice(filtered, 3)) This ensures you don’t do more work than you have to to produce those 3 elements. Note … Read more
In Python 3, you have to use the interact command in pdb before you can access any non-global variables due to a change in the way comprehensions are implemented. >>> def foo(): [][0] … >>> foo() Traceback (most recent call last): File “<stdin>”, line 1, in <module> File “<stdin>”, line 1, in foo IndexError: list … Read more
generic case using Array.map, requires javascript 1.6 (that means, works on every browser but IE < 9) or with an object augmenting framework like MooTools works on every browser: var list_of_names = document.getElementsByTagName(‘input’).map( function(element) { return element.getAttribute(‘name’); } ); If dealing with an iterable that doesn’t provide map (such as generators), spread syntax can be … 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…
[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’s just a shorter way of expressing a list. li = [row[index] for row in outer_list] is equivalent to: li = [] for row in outer_list: li.append(row[index]) Once you get used to the syntax, it becomes a tidy way of creating lists (and other iterables).
By using list comprehension, you actually define a new scope. Indeed if we alter the list comprehension to: out2 = [print(globals()) or print(locals()) or eval(cmd) for cmd in [‘self.b’]] we force Python to print the local and global variables before making the eval(..) call, and we obtain something like: {‘__builtins__’: <module ‘builtins’ (built-in)>, ‘__name__’: ‘__main__’, … Read more
append only works on variables, not list literals, since it updates the list object itself and does not return the resulting list. As @Tomalak mentioned noted, running a similar operation on a simple list also returns None >>> [1, 2, 3].append(4) == None True
No-one has given the usual answer: def flat(l): return [y for x in l for y in x] There are dupes of this question floating around StackOverflow.
>>> l = [‘a’, ‘c’, ‘e’, ‘b’] >>> [x for pair in zip(l,l) for x in pair] [‘a’, ‘a’, ‘c’, ‘c’, ‘e’, ‘e’, ‘b’, ‘b’] Or >>> from itertools import repeat >>> [x for item in l for x in repeat(item, 2)] [‘a’, ‘a’, ‘c’, ‘c’, ‘e’, ‘e’, ‘b’, ‘b’]