What are dictionary view objects?

Question

In python 2.7, we got the dictionary view methods available.

Now, I know the pro and cons of the following:

• dict.items() (and values, keys): returns a list, so you can actually store the result, and
• dict.iteritems() (and the like): returns a generator, so you can iterate over each value generated one by one.

What are dict.viewitems() (and the like) for? What are their benefits? How does it work? What is a view after all?

I read that the view is always reflecting the changes from the dictionary. But how does it behave from the perf and memory point of view? What are the pro and cons?

Answer 1

Dictionary views are essentially what their name says: views are simply like a window on the keys and values (or items) of a dictionary. Here is an excerpt from the official documentation for Python 3:

>>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500}
>>> keys = dishes.keys()
>>> values = dishes.values()

>>> # view objects are dynamic and reflect dict changes
>>> del dishes['eggs']
>>> keys  # No eggs anymore!
dict_keys(['sausage', 'bacon', 'spam'])

>>> values  # No eggs value (2) anymore!
dict_values([1, 1, 500])

(The Python 2 equivalent uses dishes.viewkeys() and dishes.viewvalues().)

This example shows the dynamic character of views: the keys view is not a copy of the keys at a given point in time, but rather a simple window that shows you the keys; if they are changed, then what you see through the window does change as well. This feature can be useful in some circumstances (for instance, one can work with a view on the keys in multiple parts of a program instead of recalculating the current list of keys each time they are needed)—note that if the dictionary keys are modified while iterating over the view, how the iterator should behave is not well defined, which can lead to errors.

One advantage is that looking at, say, the keys uses only a small and fixed amount of memory and requires a small and fixed amount of processor time, as there is no creation of a list of keys (Python 2, on the other hand, often unnecessarily creates a new list, as quoted by Rajendran T, which takes memory and time in an amount proportional to the length of the list). To continue the window analogy, if you want to see a landscape behind a wall, you simply make an opening in it (you build a window); copying the keys into a list would correspond to instead painting a copy of the landscape on your wall—the copy takes time, space, and does not update itself.

To summarize, views are simply… views (windows) on your dictionary, which show the contents of the dictionary even after it changes. They offer features that differ from those of lists: a list of keys contain a copy of the dictionary keys at a given point in time, while a view is dynamic and is much faster to obtain, as it does not have to copy any data (keys or values) in order to be created.