dictview

I've been trying to understand built-in view objects return by .items() , .values() , .keys() in Python 3 or similarly by .viewitems() , .viewvalues() , .viewkeys() . There are other threads on that subject but none (even the doc ) seems to described how they work internally. The main gain here seems to be efficienty compared to the copy of type list returned in Python 2. There are often compared to a window to the dictionnary items (like in this thread ). But what is that window and why is it more efficient ? The only thing I can see is that the view objects seems to be set-like objects,

dict methods dict.keys(), dict.items() and dict.values() return “views” instead of lists. http://docs.python.org/dev/3.0/whatsnew//3.0.html First of all how is a view different from an iterator? Secondly, what is the benefit of this change? Is it just for performance reasons? It doesn't seem intuitive to me, i.e., I'm asking for a list of thing (give me all your keys) and I'm getting something else back. Will this confuse people? You are effectively getting a list. It's just not a copy of the internal list, but something that acts as if it where a list but only represents the internal state.

In Python 3.7, I'd like to calculate the intersection of two dictionaries' keys. To do this, I'd like to call the .intersection() method on their keys() , however it does not work. .keys() produces a set-like object, however most set methods don't work on it. What works however is the extremely unknown bitwise operator overloads for set-like objects, like & . m = {'a':1, 'b':2} n = {'b':3, 'c':4} m.keys().intersection(n.keys()) # Pythonic, but doesn't work m.keys() & n.keys() # works but not readable set(m.keys()).intersection(set(n.keys())) # works, readable, but too verbose I find that the &

Note: code examples in python3, but the question stands for python2 as well (replacing .keys with .viewkeys , etc) dict objects provide view methods which (sometimes) support set operations: >>> {'a': 0, 'b': 1}.keys() & {'a'} {'a'} >>> {'a': 0, 'b': 1}.items() & {('a', 0)} {('a', 0)} But the values view does not support set operators: >>> {'a': 0, 'b': 1}.values() & {0} Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unsupported operand type(s) for &: 'dict_values' and 'set' I understand that a dict value can be a non-hashable object, so it is not always

I have this sentence: def Ciudad(prob): numero = random.random() ciudad = prob.keys()[0] for i in prob.keys(): if(numero > prob[i]): if(prob[i] > prob[ciudad]): ciudad = i else: if(prob[i] > prob[ciudad]): ciudad = i return ciudad But when I call it this error pops: TypeError: 'dict_keys' object does not support indexing is it a version problem? I'm using Python 3.3.2 dict.keys() is a dictionary view. Just use list() directly on the dictionary instead if you need a list of keys, item 0 will be the first key in the (arbitrary) dictionary order: list(prob)[0] or better still just use: next(iter