1、先看看什么是 iterable 對象

以內置的max函數為例子，查看其doc：
代碼如下:
>>> print max.__doc__
max(iterable[, key=func]) -> value
max(a, b, c, ...[, key=func]) -> value

With a single iterable argument, return its largest item.
With two or more arguments, return the largest argument.
在max函數的第一種形式中，其第一個參數是一個 iterable 對象，既然這樣，那么哪些是 iterable 對象呢？
代碼如下:
>>> max('abcx')
>>> 'x'
>>> max('1234')
>>> '4'
>>> max((1,2,3))
>>> 3
>>> max([1,2,4])
>>> 4
我們可以使用yield生成一個iterable 對象（也有其他的方式）:
代碼如下:
def my_range(start,end):
    ''' '''
    while start <= end:
        yield start
        start += 1
執行下面的代碼：
代碼如下:
for num in my_range(1, 4):
    print num
print max(my_range(1, 4))
將輸出：
代碼如下:
1
2
3
4
4


2、map

在http://docs.python.org/2/library/functions.html#map中如此介紹map函數：
代碼如下:
map(function, iterable, ...)
Apply function to every item of iterable and return a list of the results. If additional iterable arguments are passed, function must take that many arguments and is applied to the items from all iterables in parallel. If one iterable is shorter than another it is assumed to be extended with None items. If function is None, the identity function is assumed; if there are multiple arguments, map() returns a list consisting of tuples containing the corresponding items from all iterables (a kind of transpose operation). The iterable arguments may be a sequence or any iterable object; the result is always a list.
map函數使用自定義的function處理iterable中的每一個元素，將所有的處理結果以list的形式返回。例如：
代碼如下:
def func(x):
    ''' '''
    return x*x

print map(func, [1,2,4,8])
print map(func, my_range(1, 4))
運行結果是：
代碼如下:
[1, 4, 16, 64]
[1, 4, 9, 16]
也可以通過列表推導來實現：
代碼如下:
print [x*x for x in [1,2,4,8]]

3、reduce

在http://docs.python.org/2/library/functions.html#reduce中如下介紹reduce函數：
代碼如下:
reduce(function, iterable[, initializer])
Apply function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). The left argument, x, is the accumulated value and the right argument, y, is the update value from the iterable. If the optional initializer is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. If initializer is not given and iterable contains only one item, the first item is returned.