There are dictionary comprehensions in Python 2.7+, but they don’t work quite the way you’re trying. Like a list comprehension, they create a new dictionary; you can’t use them to add keys to an existing dictionary. Also, you have to specify the keys and values, although of course you can specify a dummy value if … Read more
Use a dict comprehension (Python 2.7 and later): {key: value for (key, value) in iterable} Alternatively for simpler cases or earlier version of Python, use the dict constructor, e.g.: pairs = [(‘a’, 1), (‘b’, 2)] dict(pairs) #=> {‘a’: 1, ‘b’: 2} dict([(k, v+1) for k, v in pairs]) #=> {‘a’: 2, ‘b’: 3} Given separate … Read more
John’s answer is good (that list comprehensions are better when you want to iterate over something multiple times). However, it’s also worth noting that you should use a list if you want to use any of the list methods. For example, the following code won’t work: def gen(): return (something for something in get_some_stuff()) print … Read more
You can totally do that. It’s just an ordering issue: [unicode(x.strip()) if x is not None else ” for x in row] In general, [f(x) if condition else g(x) for x in sequence] And, for list comprehensions with if conditions only, [f(x) for x in sequence if condition] Note that this actually uses a different … Read more
You can use a list comprehension with enumerate: indices = [i for i, x in enumerate(my_list) if x == “whatever”] The iterator enumerate(my_list) yields pairs (index, item) for each item in the list. Using i, x as loop variable target unpacks these pairs into the index i and the list item x. We filter down … Read more
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
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.
map may be microscopically faster in some cases (when you’re NOT making a lambda for the purpose, but using the same function in map and a listcomp). List comprehensions may be faster in other cases and most (not all) pythonistas consider them more direct and clearer. An example of the tiny speed advantage of map … Read more
Class scope and list, set or dictionary comprehensions, as well as generator expressions do not mix. The why; or, the official word on this In Python 3, list comprehensions were given a proper scope (local namespace) of their own, to prevent their local variables bleeding over into the surrounding scope (see List comprehension rebinds names … Read more
If you’re just looking to iterate over a flattened version of the data structure and don’t need an indexable sequence, consider itertools.chain and company. >>> list_of_menuitems = [[‘image00’, ‘image01’], [‘image10’], []] >>> import itertools >>> chain = itertools.chain(*list_of_menuitems) >>> print(list(chain)) [‘image00’, ‘image01’, ‘image10′] It will work on anything that’s iterable, which should include Django’s iterable … Read more