dictionary-comprehension

In python2.7+ exist any way to make something like: { something_if_true if condition else something_if_false for key, value in dict_.items() } I know you can make anything with just 'if' { something_if_true for key, value in dict_.items() if condition} Marcin You've already got it: A if test else B is a valid python expression. The only problem with your dict comprehension as shown is that the place for an expression in a dict comprehension must have two expressions, separated by a colon: { (some_key if condition else default_key):(something_if_true if condition else something_if_false) for

>>> a = 1 >>> print { key: locals()[key] for key in ["a"] } Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 1, in <dictcomp> KeyError: 'a' How can I create a dictionary with a comprehension like this? A dict comprehension has its own namespace , and locals() in that namespace has no a . Technically speaking, everything but the initial iterable for the outermost iterable (here ["a"] ) is run almost as a nested function with the outermost iterable passed in as an argument. Your code works if you used globals() instead, or created a reference to the

Trying to think of a one-liner to achieve the following ( summing all the values of a key) : >>> data = [('a',1),('b',3),('a',4),('c',9),('b',1),('d',3)] >>> res = {} >>> for tup in data: ... res[tup[0]] = res.setdefault(tup[0],0) + tup[1] ... >>> res {'a': 5, 'c': 9, 'b': 4, 'd': 3} One-liner version without using any imports like itertools,collections etc. { tup[0] : SELF_REFERENCE.setdefault(tup[0],0) + tup[1] for tup in data } Is it possible in Python to use a reference to the object currently being comprehended ? If not, is there any way to achieve this in a one-liner without using any

As we all know, there's list comprehension, like [i for i in [1, 2, 3, 4]] and there is dictionary comprehension, like {i:j for i, j in {1: 'a', 2: 'b'}.items()} but (i for i in (1, 2, 3)) will end up in a generator, not a tuple comprehension. Why is that? My guess is that a tuple is immutable, but this does not seem to be the answer. Martijn Pieters You can use a generator expression: tuple(i for i in (1, 2, 3)) but parentheses were already taken for … generator expressions. chepner Raymond Hettinger (one of the Python core developers) had this to say about tuples in a recent tweet : #python