【Python练习cookbook】迭代器、生成器

最新推荐文章于 2023-02-18 17:11:30 发布

Code_LT

最新推荐文章于 2023-02-18 17:11:30 发布

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本文链接：https://blog.youkuaiyun.com/Code_LT/article/details/107840161

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1。迭代器都可以用for循环遍历，还可以以下方式：

with open('/etc/passwd') as f:
    try:
        while True:
        line = next(f)
        print(line, end='')
    except StopIteration:
        pass

with open('/etc/passwd') as f:
    while True:
        line = next(f, None)
        if line is None:
            break
        print(line, end='')

next()函数，支持接收自定义参数来替代返回StopIteration异常

2.自定义class要使用迭代器功能，需完善__iter__()函数，使该函数能返回可迭代对象

如：

class Node:
    def __init__(self, value):
        self._value = value
        self._children = []
        def __repr__(self):
        return 'Node({!r})'.format(self._value)

    def add_child(self, node):
        self._children.append(node)

    def __iter__(self):
        return iter(self._children)
# Example
if __name__ == '__main__':
root = Node(0)
child1 = Node(1)
child2 = Node(2)
root.add_child(child1)
root.add_child(child2)
for ch in root:
    print(ch)
# Outputs Node(1), Node(2)

3. 生成器yield使用。

详情参见：https://blog.youkuaiyun.com/Code_LT/article/details/107166915

>>> def countdown(n):
...     print('Starting to count from', n)
...     while n > 0:
...         yield n
...         n -= 1
...     print('Done!')
...
>>> # Create the generator, notice no output appears
>>> c = countdown(3)
>>> c
<generator object countdown at 0x1006a0af0>
>>> # Run to first yield and emit a value
>>> next(c)
Starting to count from 3
3
>>> # Run to the next yield
>>> next(c)
2
>>> # Run to next yield
>>> next(c)
1
>>> # Run to next yield (iteration stops)
>>> next(c)
Done!
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
>>>

next()作用下，到停止时，会报StopIteration错误，for循环的话会自动处理这个异常，不需要担心。

4. yield from用法

详见：https://www.jianshu.com/p/87da832730f5

如果生成器函数需要产出另一个生成器生成的值，传统的解决方法是使用嵌套的for循环：

>>> def chain(*iterables):
...     for it in iterables:
...         for i in it:
...             yield i
>>> s = 'ABC'
>>> t = tuple(range(3))
>>> list(chain(s, t))
['A', 'B', 'C', 0, 1, 2]

chain 生成器函数把操作依次交给接收到的各个可迭代对象处理。

Python3.3之后引入了新语法：
>>> def chain(*iterables):
...     for i in iterables:
...         yield from i
...
>>> list(chain(s, t))
['A', 'B', 'C', 0, 1, 2]

yield from 相当于用来替代 for 循环。
yield from x 表达式对 x 对象所做的第一件事是，调用 iter(x)，从中获取迭代器。因
此，x 可以是任何可迭代的对象。

使用示例：

class Node:
    def __init__(self, value):
        self._value = value
        self._children = []

    def __repr__(self):
        return 'Node({!r})'.format(self._value)

    def add_child(self, node):
        self._children.append(node)

    def __iter__(self):
        return iter(self._children)

    def depth_first(self): #先看下面两个yield的示例
        yield self
        for c in self:
            yield from c.depth_first()


if __name__ == '__main__':
    root = Node(0)
    child1 = Node(1)
    child2 = Node(2)
    root.add_child(child1)
    root.add_child(child2)
    child1.add_child(Node(3))
    child1.add_child(Node(4))
    child2.add_child(Node(5))
    for ch in root.depth_first():
        print(ch)

# Outputs Node(0), Node(1), Node(3), Node(4), Node(2), Node(5)

第二个yield也会返回

def test():
    for i in range(0,4):
        yield i
        yield i+1
        
t=test()
next(t)
Out[4]: 0
next(t)
Out[5]: 1
next(t)
Out[6]: 1
next(t)
Out[7]: 2
next(t)
Out[8]: 2

5. 当reversed(a)，当a不能直接使用，需要先转成list时（耗费性能），可构造a的__reversed__()函数，实现逆序

Reversed iteration only works if the object in question has a size that can be determined or if the object implements a __reversed__() special method.

>>> a = [1, 2, 3, 4]
>>> for x in reversed(a):
... print(x)

6. 可在__iter__()函数中做其他操作，比如记录iter的某段过程：

def __iter__(self):
    for lineno, line in enumerate(self.lines,1):
        self.history.append((lineno, line)) #history为deque
        yield line

deque详解：https://zhuanlan.zhihu.com/p/32201189

7. Iterator 和 generator 不能直接切片，但可用如下方式：

>>> import itertools
>>> for x in itertools.islice(c, 10, 20):
... print(x)
...

isslice()会返回一个iterator，但会消费掉原有Iterator和 generator，使得c不能被二次使用。

8. 跳过 Iterator 和 generator 的某些值

8.1 dropewhile(表达式，iterable)：抛弃表达式为True的值，返回 Iterator

>>> from itertools import dropwhile
>>> with open('/etc/passwd') as f:
...     for line in dropwhile(lambda line: line.startswith('#'), f):
...         print(line, end='')

8.2 知道确切扔掉的数量时：

>>> from itertools import islice
>>> items = ['a', 'b', 'c', 1, 4, 10, 15]
>>> for x in islice(items, 3, None):
...     print(x)
...
1
4
10
15
>>>

9.enumerate() 返回iterator

>>> my_list = ['a', 'b', 'c']
>>> for idx, val in enumerate(my_list):
... print(idx, val)
...
0 a
1 b
2 c

>>> my_list = ['a', 'b', 'c']
>>> for idx, val in enumerate(my_list, 1):
... print(idx, val)
...
1 a
2 b
3 c

10. zip() 返回iterator

>>> a = [1, 2, 3]
>>> b = ['w', 'x', 'y', 'z']
>>> for i in zip(a,b):
... print(i)
...
(1, 'w')
(2, 'x')
(3, 'y')
>>>



>>> from itertools import zip_longest
>>> for i in zip_longest(a,b):
... print(i)
...
(1, 'w')
(2, 'x')
(3, 'y')
(None, 'z')
>>> for i in zip_longest(a, b, fillvalue=0):
... print(i)
...
(1, 'w')
(2, 'x')
(3, 'y')
(0, 'z')


>>> a = [1, 2, 3]
>>> b = [10, 11, 12]
>>> c = ['x','y','z']
>>> for i in zip(a, b, c):
... print(i)
...
(1, 10, 'x')
(2, 11, 'y')
(3, 12, 'z')

11. chain() 用于遍历多个容器更高效，因为直接生成iterator 消费iterable输入。

>>> from itertools import chain
>>> a = [1, 2, 3, 4]
>>> b = ['x', 'y', 'z']
>>> for x in chain(a, b):
... print(x)
...
1
2
3
4
x
y
z

# 低效，因为+要先生成新序列
for x in a + b:
    ...
#高效
for x in chain(a, b):
    ...

12. 利用生成器构建pipeline，流式处理大批量数据，防止内存溢出

import os
import fnmatch
import gzip
import bz2
import re


def gen_find(filepat, top):
    '''
    Find all filenames in a directory tree that match a shell wildcard pattern
    '''
    for path, dirlist, filelist in os.walk(top):
        for name in fnmatch.filter(filelist, filepat):
            yield os.path.join(path, name)


def gen_opener(filenames):
    '''
    Open a sequence of filenames one at a time producing a file object.
    The file is closed immediately when proceeding to the next iteration.
    '''

    for filename in filenames:
        if filename.endswith('.gz'):
            f = gzip.open(filename, 'rt')
        elif filename.endswith('.bz2'):
            f = bz2.open(filename, 'rt')
        else:
            f = open(filename, 'rt')
        yield f
        f.close()


def gen_concatenate(iterators):
    '''
    Chain a sequence of iterators together into a single sequence.
    '''

    for it in iterators:
        yield from it


def gen_grep(pattern, lines):
    '''
    Look for a regex pattern in a sequence of lines
    '''

    pat = re.compile(pattern)
    for line in lines:
        if pat.search(line):
            yield line

#使用：寻找含有python字串的line
lognames = gen_find('access-log*', 'www')
files = gen_opener(lognames)
lines = gen_concatenate(files)
pylines = gen_grep('(?i)python', lines)
for line in pylines:
    print(line)

13. 用迭代器替代有结束条件的无限循环

CHUNKSIZE = 8192
def reader(s):
    while True:
        data = s.recv(CHUNKSIZE)
        if data == b'':
            break
        process_data(data

可替换为：

def reader(s):
    for chunk in iter(lambda: s.recv(CHUNKSIZE), b''):
        process_data(data)

原因是iter()可接收一个不带参数的函数和一个结束值，迭代器反复调用该函数直到返回值为结束值时停止。

>>> import sys
>>> f = open('/etc/passwd')
>>> for chunk in iter(lambda: f.read(10), ''):
... n = sys.stdout.write(chunk)