The range Iterator

We studied the range built-in’s basic behavior in the prior chapter. In 3.0, it returns an iterator that generates numbers in the range on demand, instead of building the result list in memory. This subsumes the older 2.X xrange (see the upcoming version skew note), and you must use list(range(...)) to force an actual range list if one is needed (e.g., to display results):

C:\\misc> c:\python30\python
>>> R = range(10)                # range returns an iterator, not a list
>>> R
range(0, 10)

>>> I = iter(R)                  # Make an iterator from the range
>>> next(I)                      # Advance to next result
0                                # What happens in for loops, comprehensions, etc.
>>> next(I)
1
>>> next(I)
2

>>> list(range(10))              # To force a list if required
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

Unlike the list returned by this call in 2.X, range objects in 3.0 support only iteration, indexing, and the len function. They do not support any other sequence operations (use list(...) if you require more list tools):

>>> len(R)                       # range also does len and indexing, but no others
10
>>> R[0]
0
>>> R[-1]
9

>>> next(I)                      # Continue taking from iterator, where left off
3
>>> I.__next__()                 # .next() becomes .__next__(), but use new next()
4

Note

Version skew note: Python 2.X also has a built-in called xrange, which is like range but produces items on demand instead of building a list of results in memory all at once. Since this is exactly what the new iterator-based range does in Python 3.0, xrange is no longer available in 3.0—it has been subsumed. You may still see it in 2.X code, though, especially since range builds result lists there and so is not as efficient in its memory usage. As noted in a sidebar in the prior chapter, the file.xreadlines() method used to minimize memory use in 2.X has been dropped in Python 3.0 for similar reasons, in favor of file iterators.