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.

range()函式返回的是一個列表，函式形式為range(start, stop[, step])
例如：

In [4] : range(5)
Out[4]: [0, 1, 2, 3, 4]
In [5]: type(range(5))
Out[5]: list

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

xrange()函式返回的是一個生成器，函式形式為xrange(start, stop[, step])
例如：

In [6]: xrange(5)
Out[6]: xrange(5)
In [7]: type(xrange(5))
Out[7]: xrange

  在迴圈中需要生成一個很大的序列時，儘量使用xrange()，該函式不用一上來就申請很大的空間在儲存元素，大大的節省的空間。

  而range()直接生成一個列表，需要很大的空間來儲存，沒有使用xrange()好。