python range() 和xrange()的區別

Help on built-in function range in module __builtin__:

range(...)
    range(stop) -> list of integers
    range(start, stop[, step]) -> list of integers
    
    Return a list containing an arithmetic progression of integers.
    range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
    When step is given, it specifies the increment (or decrement).
    For example, range(4) returns [0, 1, 2, 3].  The end point is omitted!
    These are exactly the valid indices for a list of 4 elements.
(END)

以上是range函數的說明，三個參數，分別代表開始，結束位置和步長。

使用方法如下：

In [2]: range(10)
Out[2]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

In [3]: range(1, 10)
Out[3]: [1, 2, 3, 4, 5, 6, 7, 8, 9]

In [4]: range(1, 10, 2)
Out[4]: [1, 3, 5, 7, 9]

In [5]: type(range(1, 3))
Out[5]: list

In [6]: for i in range(10):
   ...:     print i
   ...:     
0
1
2
3
4
5
6
7
8
9

　　

而xrange（）說明如下：

Help on class xrange in module __builtin__:

class xrange(object)
 |  xrange(stop) -> xrange object
 |  xrange(start, stop[, step]) -> xrange object
 |  
 |  Like range(), but instead of returning a list, returns an object that
 |  generates the numbers in the range on demand.  For looping, this is 
 |  slightly faster than range() and more memory efficient.
 |  
 |  Methods defined here:
 |  
 |  __getattribute__(...)
 |      x.__getattribute__('name') <==> x.name
 |  
 |  __getitem__(...)
 |      x.__getitem__(y) <==> x[y]
 |  
 |  __iter__(...)
 |      x.__iter__() <==> iter(x)
 |  
 |  __len__(...)
 |      x.__len__() <==> len(x)
 |  
 |  __reduce__(...)
 |  
 |  __repr__(...)
 |      x.__repr__() <==> repr(x)
 |  
 |  __reversed__(...)
 |      Returns a reverse iterator.
 |  
 |  ----------------------------------------------------------------------
 |  Data and other attributes defined here:
 |  
 |  __new__ = <built-in method __new__ of type object>
 |      T.__new__(S, ...) -> a new object with type S, a subtype of T
(END)

說明了兩者的區別是xrange返回的是一個可迭代的對象，range返回的則是一個列表. 同時效率更高，更快。

原因是實現的時候使用了yield（唔，源碼沒看見，具體對比可以看一下http://ju.outofmemory.cn/entry/122781），

因此更節省內存，規模越大區別更明顯.

關於可迭代對象的定義見：https://eastlakeside.gitbooks.io/interpy-zh/content/Generators/Iterable.html

即，只要定義了可以返回一個迭代器的__iter__方法，或者__getitem__那么就是可迭代對象。

 

xrange使用方法如下：

In [13]: list(xrange(3))
Out[13]: [0, 1, 2]

In [14]: a = xrange(3)

In [15]: a.__iter__
Out[15]: <method-wrapper '__iter__' of xrange object at 0x7f415be1bdc8>

In [16]: a.__iter__()
Out[16]: <rangeiterator at 0x7f415aa27210>