浅谈python内置变量

 代码如下,那么哪些是 iterable 对象呢

1、简单解释就是:反转一个序列对象

1、先看看什么是 iterable 对象

例子1:

以内置的max函数为例子,查看其doc:

 

复制代码 代码如下:

 代码如下

>>> print max.__doc__
max(iterable[, key=func]) -> value
max(a, b, c, …[, key=func]) -> value

deffun3():

With a single iterable argument, return its largest item.
With two or more arguments, return the largest argument.

  x=[3,6,9]

在max函数的第一种形式中,其第一个参数是一个 iterable
对象,既然这样,那么哪些是 iterable 对象呢?

  foriinreversed(x):

复制代码 代码如下:

  print(i,end=’,’)

>>> max(‘abcx’)
>>> ‘x’
>>> max(‘1234’)
>>> ‘4’
>>> max((1,2,3))
>>> 3
>>> max([1,2,4])
>>> 4

fun3()

我们可以使用yield生成一个iterable 对象(也有其他的方式):

 

复制代码 代码如下:

输出:

def my_range(start,end):
    ”’ ”’
    while start <= end:
        yield start
        start += 1

》》》9,6,3,

执行下面的代码:

例子2:

复制代码 代码如下:

 

for num in my_range(1, 4):
    print num
print max(my_range(1, 4))

 代码如下

将输出:

>>> a=range(5)

复制代码 代码如下:

>>> a

1
2
3
4
4

range(0,5)

2、map

>>>list(a)

[0,1,2,3,4]

复制代码 代码如下:

>>> a1=reversed(a)

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.

>>>list(a1)

map函数使用自定义的function处理iterable中的每一个元素,将所有的处理结果以list的形式返回。例如:

[4,3,2,1,0]

复制代码 代码如下:

 

def func(x):
    ”’ ”’
    return x*x

print map(func, [1,2,4,8])
print map(func, my_range(1, 4))

运行结果是:

复制代码 代码如下:

[1, 4, 16, 64]
[1, 4, 9, 16]

也可以通过列表推导来实现:

复制代码 代码如下:

print [x*x for x in [1,2,4,8]]

3、reduce

复制代码 代码如下:

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.

这个已经介绍的很明了,

复制代码 代码如下:

reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])

相当于计算

复制代码 代码如下:

((((1+2)+3)+4)+5)

而:

复制代码 代码如下:

reduce(lambda x, y: x+y, [1, 2, 3, 4, 5],6)

相当于计算

复制代码 代码如下:

(((((6+1)+2)+3)+4)+5)

4、filter

复制代码 代码如下:

filter(function, iterable)
Construct a list from those elements of iterable for which function
returns true. iterable may be either a sequence, a container which
supports iteration, or an iterator. If iterable is a string or a tuple,
the result also has that type; otherwise it is always a list. If
function is None, the identity function is assumed, that is, all
elements of iterable that are false are removed.

Note that filter(function, iterable) is equivalent to [item for item in
iterable if function(item)] if function is not None and [item for item
in iterable if item] if function is None.

参数function(是函数)用于处理iterable中的每个元素,如果function处理某元素时候返回true,那么该元素将作为list的成员而返回。比如,过滤掉字符串中的字符a:

复制代码 代码如下:

def func(x):
    ”’ ”’
    return x != ‘a’

print filter(func, ‘awake’)

运行结果是:

复制代码 代码如下:

wke

这也可以通过列表推导来实现:

复制代码 代码如下:

print ”.join([x for x in ‘awake’ if x != ‘a’])

iterable 对象
以内置的max函数为例子,查看其doc: 复制代码 代码如下: print
max.__doc__ max(iterable[, key=func]) – value max(a, b,
c,…