Suppose you have a text full of sentences and you want an array of words. # Without list comprehension list_of_words = [] for sentence in text: for word in sentence: list_of_words.append(word) return list_of_words I like to think of list comprehension as stretching code horizontally. Try breaking it up into: # List Comprehension [word for sentence … 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
You can use a generator expression: tuple(i for i in (1, 2, 3)) but parentheses were already taken for … generator expressions.
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
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
>>> xs = [22, 13, 45, 50, 98, 69, 43, 44, 1] >>> [x+1 if x >= 45 else x+5 for x in xs] [27, 18, 46, 51, 99, 70, 48, 49, 6] Do-something if <condition>, else do-something else.
You can also write: [e] * n You should note that if e is for example an empty list you get a list with n references to the same list, not n independent empty lists. Performance testing At first glance it seems that repeat is the fastest way to create a list with n identical … Read more
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
You can totally do that. It’s just an ordering issue: [f(x) if x is not None else ” for x in xs] 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
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