New Iterables in Python 3.0

One of the fundamental changes in Python 3.0 is that it has a stronger emphasis on iterators than 2.X. In addition to the iterators associated with built-in types such as files and dictionaries, the dictionary methods keys, values, and items return iterable objects in Python 3.0, as do the built-in functions range, map, zip, and filter. As shown in the prior section, the last three of these functions both return iterators and process them. All of these tools produce results on demand in Python 3.0, instead of constructing result lists as they do in 2.6.

Although this saves memory space, it can impact your coding styles in some contexts. In various places in this book so far, for example, we’ve had to wrap up various function and method call results in a list(...) call in order to force them to produce all their results at once:

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>>> zip('abc', 'xyz')                  # An iterable in Python 3.0 (a list in 2.6)
<zip object at 0x02E66710>

>>> list(zip('abc', 'xyz'))            # Force list of results in 3.0 to display
[('a', 'x'), ('b', 'y'), ('c', 'z')]

This isn’t required in 2.6, because functions like zip return lists of results. In 3.0, though, they return iterable objects, producing results on demand. This means extra typing is required to display the results at the interactive prompt (and possibly in some other contexts), but it’s an asset in larger programs—delayed evaluation like this conserves memory and avoids pauses while large result lists are computed. Let’s take a quick look at some of the new 3.0 iterables in action.