来自 @nedbat 的分享:

Ned Batchelder: Loop Like A Native

迭代基础

C样:

i = 0
while i < len(my_list):
    v = my_list[i]
    print v
    i += 1

偏门:

for i in range(len(my_list)):
    v = my_list[i]
    print v

蠎样:

for v in my_list:
    print v

模式:

for name in iterable:
    statements
  • iterable 的对象可以生成值流
  • 对迭代对象的每次操作
  • 由对象决定是什么值
  • Python 中太多对象可以直接迭代的…

list ⇒ elements

for e in [1, 2, 3, 4]:
    print e

1
2
3
4

Strings ⇒ characters

for c in "Hello":
    print c

H
e
l
l
o

Dicts ⇒ keys

d = { 'a': 1,  'b': 2,  'c': 3 }

for k in d:
    print k

a
c
b
  • 当然顺序是没有的
  • 以及:
    • for v in d.itervalues():
    • for k,v in d.iteritems():
    • 专用形式

Files ⇒ lines

with open("gettysburg.txt") as f:
    for line in f:
        print repr(line)

'Four-score and seven years ago,\n'
'our fathers brought forth on this continent\n'
'a new nation,\n'
'conceived in liberty,\n'
'and dedicated to the proposition\n'
'that all men are created equal.\n'

标准库中有趣的迭代

正则表达式:

for match in re.finditer(pattern, string):
    # once for each regex match...

文件系统:

for root, dirs, files in os.walk('/some/dir'):
    # once for each sub-directory...

迭代工具:

for num in itertools.count():
    # once for each integer... Infinite!

from itertools import chain, repeat, cycle
seq = chain(repeat(17, 3), cycle(range(4)))
for num in seq:
    # 17, 17, 17, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, ...

其它的迭代形式

new_list = list(iterable)

results = [f(x) for x in iterable]

total = sum(iterable)

smallest = min(iterable)
largest = max(iterable)

combined = "".join(iterable)

基本设问

Q: 如何获得索引号?

别:

for i in range(len(my_list)):
    v = my_list[i]
    print i, v

赞:

for i, v in enumerate(my_list):
    print i, v

enumerate() 制造好用的值对

names = ["Eiffel Tower", "Empire State", "Sears Tower"]
list(enumerate(names))

[(0, 'Eiffel Tower'), 
 (1, 'Empire State'), 
 (2, 'Sears Tower')]

for num, name in enumerate(names):
    print num, name

0 Eiffel Tower
1 Empire State
2 Sears Tower

迭代 vs 索引

受限:

for i in range(len(my_list)):
    v = my_list[i]    # indexing!
    print i, v

更强:

for i, v in enumerate(iterable):
    print i, v

for linenum, line in enumerate(f, start=1):
    #...

C样坏形:

i = 0
for v in iterable:
    print i, v
    i += 1

Q: 如何对两个列表循环?

names = ["Eiffel Tower", "Empire State", "Sears Tower"]
heights = [324, 381, 442]

for i in range(len(names)):
    name = names[i]
    height = heights[i]
    print "%s: %s meters" % (name, height)

Eiffel Tower: 324 meters
Empire State: 381 meters
Sears Tower: 442 meters

zip() 生成弱对关系循环

将一对循环,变成一个流循环:

for name, height in zip(names, heights):
    print "%s: %s meters" % (name, height)

Eiffel Tower: 324 meters
Empire State: 381 meters
Sears Tower: 442 meters

dict() 是接收一对流的

names = ["Eiffel Tower", "Empire State", "Sears Tower"]
heights = [324, 381, 442]

dict(zip(names, heights))

{'Empire State': 381, 
 'Sears Tower': 442, 
 'Eiffel Tower': 324}

最赞

tall_buildings = {
  "Empire State": 381, "Sears Tower": 442,
  "Burj Khalifa": 828, "Taipei 101": 509,
  }

>>> print max(tall_buildings.values())
828

>>> print max(tall_buildings.items(), key=lambda b: b[1])
('Burj Khalifa', 828)

>>> print max(tall_buildings, key=tall_buildings.get)
'Burj Khalifa'

自制迭代

生成自有迭代

nums = [88, 73, 92, 72, 40, 38, 25, 20, 90, 72]
for n in nums:
    if n % 2 == 0:
        do_something(n)

def evens(stream):
    them = []
    for n in stream:
        if n % 2 == 0:
            them.append(n)
    return them

for n in evens(nums):
    do_something(n)

生成器

函式返回一个值 —> 生成器生成一个流

def hello_world():
    yield "Hello"
    yield "world"

for x in hello_world():
    print x

Hello
world

Evens generator

def evens(stream):
    for n in stream:
        if n % 2 == 0:
            yield n

for n in evens(nums):
    do_something(n)

对迭代提炼

~ Abstracting your iteration

f = open("my_config.ini")
for line in f:
    line = line.strip()
    if line.startswith('#'):
        # A comment line, skip it.
        continue
    if not line:
        # A blank line, skip it.
        continue

    # An interesting line.
    do_something(line)

自制生成器:

def interesting_lines(f):
    for line in f:
        line = line.strip()
        if line.startswith('#'):
            continue
        if not line:
            continue
        yield line

with open("my_config.ini") as f:
    for line in interesting_lines(f):
        do_something(line)

with open("my_other.dat") as f2:
    for line in interesting_lines(f2):
        do_something_else(line)

Q: 如何从两层循环中退出?

for row in range(height):
    for col in range(width):

        value = spreadsheet.get_value(col, row)
        do_something(value)

        if this_is_my_value(value):
            break   #  ???

A: 合并循环

def range_2d(width, height):
    """Produce a stream of two-D coordinates."""
    for y in range(height):
        for x in range(width):
            yield x, y

for col, row in range_2d(width, height):
    value = spreadsheet.get_value(col, row)
    do_something(value)

    if this_is_my_value(value):
        break

Better: 单元迭代

for cell in spreadsheet.cells():
    value = cell.get_value()
    do_something(value)

    if this_is_my_value(value):
        break

底层迭代

  • 可迭代: 能生成迭代 Iterable: produces an iterator
  • 迭代器: 生成一个值流 Iterator: produces a stream of values

    iterator = iter(iterable) # iterable.iter() value = next(iterator) # iterator.next() or .next() value = next(iterator) …

迭代器仅有一个操作: next()

迭代基层操作

有时很有用:

with open("blah.dat") as f:
    # Read the first line
    header_line = next(f)

    # Read the rest
    for data_line in f:
        # ...

构造自制对象可迭代

class ToDoList(object):
    def __init__(self):
        self.tasks = []

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

todo = ToDoList()
...
for task in todo:
    # ...
  • 仅仅需要定义 __iter__()

iter 生成器

class ToDoList(object):
    def __init__(self):
        self.tasks = []

    def __iter__(self):
        for task in self.tasks:
            if not task.done:
                yield task

    def all(self):
        return iter(self.tasks)

    def done(self):
        return (t for t in self.tasks if t.done)

( ̄▽ ̄)

  • 迭代无处不在
  • Python 内置了干净强大的模型支持迭代
  • 抽象和定制你的迭代吧!

是也乎

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  • 160531 动念

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