30个有关Python的小技巧

2014/04/01 | 分类： PYTHON, 开发 | 3 条评论 | 标签： PYTHON

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本文由 伯乐在线 - Kevin Sun 翻译自 sahandsaba。欢迎加入技术翻译小组。转载请参见文章末尾处的要求。

从我开始学习python的时候，我就开始自己总结一个python小技巧的集合。后来当我什么时候在Stack Overflow
或者在某个开源软件里看到一段很酷代码的时候，我就很惊讶：原来还能这么做！，当时我会努力的自己尝试一下这段代码，直到我懂了它的整体思路以后，我就把这段代码加到我的集合里。这篇博客其实就是这个集合整理后一部分的公开亮相。如果你已经是个python大牛，那么基本上你应该知道这里面的大多数用法了，但我想你应该也能发现一些你不知道的新技巧。而如果你之前是一个c,c++,java的程序员，同时在学习python，或者干脆就是一个刚刚学习编程的新手，那么你应该会看到很多特别有用能让你感到惊奇的实用技巧，就像我当初一样。

每一个技巧和语言用法都会在一个个实例中展示给大家，也不需要有其他的说明。我已经尽力把每个例子弄的通俗易懂，但是因为读者对python的熟悉程度不同，仍然可能难免有一些晦涩的地方。所以如果这些例子本身无法让你读懂，至少这个例子的标题在你后面去google搜索的时候会帮到你。

整个集合大概是按照难易程度排序，简单常见的在前面，比较少见的在最后。

1.1 拆箱

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>>> a, b, c =1,2,3

>>> a, b, c

(1,2,3)

>>> a, b, c =[1,2,3]

>>> a, b, c

(1,2,3)

>>> a, b, c =(2* i +1 for i inrange(3))

>>> a, b, c

(1,3,5)

>>> a, (b, c), d =[1, (2,3),4]

>>> a

1

>>> b

2

>>> c

3

>>> d

4

1.2 拆箱变量交换

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>>> a, b =1,2

>>> a, b =b, a

>>> a, b

(2,1)

1.3 扩展拆箱（只兼容python3）

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>>> a, *b, c =[1,2,3,4,5]

>>> a

1

>>> b

[2,3,4]

>>> c

5

1.4 负数索引

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>>> a =[0,1,2,3,4,5,6,7,8,9,10]

>>> a[-1]

10

>>> a[-3]

8

1.5 切割列表

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>>> a =[0,1,2,3,4,5,6,7,8,9,10]

>>> a[2:8]

[2,3,4,5,6,7]

1.6 负数索引切割列表

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>>> a =[0,1,2,3,4,5,6,7,8,9,10]

>>> a[-4:-2]

[7,8]

1.7指定步长切割列表

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>>> a =[0,1,2,3,4,5,6,7,8,9,10]

>>> a[::2]

[0,2,4,6,8,10]

>>> a[::3]

[0,3,6,9]

>>> a[2:8:2]

[2,4,6]

1.8 负数步长切割列表

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>>> a =[0,1,2,3,4,5,6,7,8,9,10]

>>> a[::-1]

[10,9,8,7,6,5,4,3,2,1,0]

>>> a[::-2]

[10,8,6,4,2,0]

1.9 列表切割赋值

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>>> a =[1,2,3,4,5]

>>> a[2:3]=[0,0]

>>> a

[1,2,0,0,4,5]

>>> a[1:1]=[8,9]

>>> a

[1,8,9,2,0,0,4,5]

>>> a[1:-1]=[]

>>> a

[1,5]

1.10 命名列表切割方式

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>>> a =[0,1,2,3,4,5]

>>> LASTTHREE =slice(-3,None)

>>> LASTTHREE

slice(-3,None,None)

>>> a[LASTTHREE]

[3,4,5]

1.11 列表以及迭代器的压缩和解压缩

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>>> a =[1,2,3]

>>> b =['a','b','c']

>>> z =zip(a, b)

>>> z

[(1,'a'), (2,'b'), (3,'c')]

>>>zip(*z)

[(1,2,3), ('a','b','c')]

1.12 列表相邻元素压缩器

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>>> a =[1,2,3,4,5,6]

>>>zip(*([iter(a)]*2))

[(1,2), (3,4), (5,6)]

 

>>> group_adjacent =lambda a, k: zip(*([iter(a)]*k))

>>> group_adjacent(a, 3)

[(1,2,3), (4,5,6)]

>>> group_adjacent(a, 2)

[(1,2), (3,4), (5,6)]

>>> group_adjacent(a, 1)

[(1,), (2,), (3,), (4,), (5,), (6,)]

 

>>>zip(a[::2], a[1::2])

[(1,2), (3,4), (5,6)]

 

>>>zip(a[::3], a[1::3], a[2::3])

[(1,2,3), (4,5,6)]

 

>>> group_adjacent =lambda a, k: zip(*(a[i::k]fori inrange(k)))

>>> group_adjacent(a, 3)

[(1,2,3), (4,5,6)]

>>> group_adjacent(a, 2)

[(1,2), (3,4), (5,6)]

>>> group_adjacent(a, 1)

[(1,), (2,), (3,), (4,), (5,), (6,)]

1.13 在列表中用压缩器和迭代器滑动取值窗口

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>>>defn_grams(a, n):

...     z =[iter(a[i:])fori inrange(n)]

...    returnzip(*z)

...

>>> a =[1,2,3,4,5,6]

>>> n_grams(a, 3)

[(1,2,3), (2,3,4), (3,4,5), (4,5,6)]

>>> n_grams(a, 2)

[(1,2), (2,3), (3,4), (4,5), (5,6)]

>>> n_grams(a, 4)

[(1,2,3,4), (2,3,4,5), (3,4,5,6)]

1.14 用压缩器反转字典

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>>> m ={'a':1,'b':2,'c':3,'d':4}

>>> m.items()

[('a',1), ('c',3), ('b',2), ('d',4)]

>>>zip(m.values(), m.keys())

[(1,'a'), (3,'c'), (2,'b'), (4,'d')]

>>> mi =dict(zip(m.values(), m.keys()))

>>> mi

{1:'a',2:'b',3:'c',4:'d'}

1.15 列表展开

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>>> a =[[1,2], [3,4], [5,6]]

>>>list(itertools.chain.from_iterable(a))

[1,2,3,4,5,6]

 

>>>sum(a, [])

[1,2,3,4,5,6]

 

>>> [x forl ina forx inl]

[1,2,3,4,5,6]

 

>>> a =[[[1,2], [3,4]], [[5,6], [7,8]]]

>>> [x forl1 ina forl2 inl1 forx inl2]

[1,2,3,4,5,6,7,8]

 

>>> a =[1,2, [3,4], [[5,6], [7,8]]]

>>> flatten =lambda x: [y forl inx fory inflatten(l)] iftype(x)islist else [x]

>>> flatten(a)

[1,2,3,4,5,6,7,8]

1.16 生成器表达式

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>>> g =(x **2 for x inxrange(10))

>>>next(g)

0

>>>next(g)

1

>>>next(g)

4

>>>next(g)

9

>>>sum(x**3 for x inxrange(10))

2025

>>>sum(x**3 for x inxrange(10)ifx %3 ==1)

408

1.17 字典推导

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>>> m ={x: x **2 for x inrange(5)}

>>> m

{0:0,1:1,2:4,3:9,4:16}

 

>>> m ={x: 'A'+ str(x)forx inrange(10)}

>>> m

{0:'A0',1:'A1',2:'A2',3:'A3',4:'A4',5:'A5',6:'A6',7:'A7',8:'A8',9:'A9'}

1.18 用字典推导反转字典

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>>> m ={'a':1,'b':2,'c':3,'d':4}

>>> m

{'d':4,'a':1,'b':2,'c':3}

>>> {v: k fork, v inm.items()}

{1:'a',2:'b',3:'c',4:'d'}

1.19 命名元组

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>>> Point =collections.namedtuple('Point', ['x','y'])

>>> p =Point(x=1.0, y=2.0)

>>> p

Point(x=1.0, y=2.0)

>>> p.x

1.0

>>> p.y

2.0

1.20 继承命名元组

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>>>classPoint(collections.namedtuple('PointBase', ['x','y'])):

...     __slots__ =()

...    def__add__(self, other):

...            returnPoint(x=self.x+other.x, y=self.y+other.y)

...

>>> p =Point(x=1.0, y=2.0)

>>> q =Point(x=2.0, y=3.0)

>>> p +q

Point(x=3.0, y=5.0)

1.21 操作集合

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>>> A ={1,2,3,3}

>>> A

set([1,2,3])

>>> B ={3,4,5,6,7}

>>> B

set([3,4,5,6,7])

>>> A | B

set([1,2,3,4,5,6,7])

>>> A & B

set([3])

>>> A -B

set([1,2])

>>> B -A

set([4,5,6,7])

>>> A ^ B

set([1,2,4,5,6,7])

>>> (A ^ B) ==((A -B) | (B -A))

True

1.22 操作多重集合

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>>> A =collections.Counter([1,2,2])

>>> B =collections.Counter([2,2,3])

>>> A

Counter({2:2,1:1})

>>> B

Counter({2:2,3:1})

>>> A | B

Counter({2:2,1:1,3:1})

>>> A & B

Counter({2:2})

>>> A +B

Counter({2:4,1:1,3:1})

>>> A -B

Counter({1:1})

>>> B -A

Counter({3:1})

1.23 统计在可迭代器中最常出现的元素

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>>> A =collections.Counter([1,1,2,2,3,3,3,3,4,5,6,7])

>>> A

Counter({3:4,1:2,2:2,4:1,5:1,6:1,7:1})

>>> A.most_common(1)

[(3,4)]

>>> A.most_common(3)

[(3,4), (1,2), (2,2)]

1.24 两端都可操作的队列

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>>> Q =collections.deque()

>>> Q.append(1)

>>> Q.appendleft(2)

>>> Q.extend([3,4])

>>> Q.extendleft([5,6])

>>> Q

deque([6,5,2,1,3,4])

>>> Q.pop()

4

>>> Q.popleft()

6

>>> Q

deque([5,2,1,3])

>>> Q.rotate(3)

>>> Q

deque([2,1,3,5])

>>> Q.rotate(-3)

>>> Q

deque([5,2,1,3])

1.25 有最大长度的双端队列

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>>> last_three =collections.deque(maxlen=3)

>>>fori inxrange(10):

...     last_three.append(i)

...    print', '.join(str(x)forx inlast_three)

...

0

0,1

0,1,2

1,2,3

2,3,4

3,4,5

4,5,6

5,6,7

6,7,8

7,8,9

1.26 可排序词典

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>>> m =dict((str(x), x) forx inrange(10))

>>>print', '.join(m.keys())

1,0,3,2,5,4,7,6,9,8

>>> m =collections.OrderedDict((str(x), x) forx inrange(10))

>>>print', '.join(m.keys())

0,1,2,3,4,5,6,7,8,9

>>> m =collections.OrderedDict((str(x), x) forx inrange(10,0,-1))

>>>print', '.join(m.keys())

10,9,8,7,6,5,4,3,2,1

1.27 默认词典

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>>> m =dict()

>>> m['a']

Traceback (most recent call last):

  File"<stdin>", line 1,in<module>

KeyError:'a'

>>>

>>> m =collections.defaultdict(int)

>>> m['a']

0

>>> m['b']

0

>>> m =collections.defaultdict(str)

>>> m['a']

''

>>> m['b']+='a'

>>> m['b']

'a'

>>> m =collections.defaultdict(lambda:'[default value]')

>>> m['a']

'[default value]'

>>> m['b']

'[default value]'

1.28 默认字典的简单树状表达

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>>>importjson

>>> tree =lambda: collections.defaultdict(tree)

>>> root =tree()

>>> root['menu']['id']='file'

>>> root['menu']['value']='File'

>>> root['menu']['menuitems']['new']['value']='New'

>>> root['menu']['menuitems']['new']['onclick']='new();'

>>> root['menu']['menuitems']['open']['value']='Open'

>>> root['menu']['menuitems']['open']['onclick']='open();'

>>> root['menu']['menuitems']['close']['value']='Close'

>>> root['menu']['menuitems']['close']['onclick']='close();'

>>>printjson.dumps(root, sort_keys=True, indent=4, separators=(',',': '))

{

    "menu": {

        "id":"file",

        "menuitems": {

            "close": {

                "onclick":"close();",

                "value":"Close"

            },

            "new": {

                "onclick":"new();",

                "value":"New"

            },

            "open": {

                "onclick":"open();",

                "value":"Open"

            }

        },

        "value":"File"

    }

}

1.29 对象到唯一计数的映射

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>>>importitertools, collections

>>> value_to_numeric_map =collections.defaultdict(itertools.count().next)

>>> value_to_numeric_map['a']

0

>>> value_to_numeric_map['b']

1

>>> value_to_numeric_map['c']

2

>>> value_to_numeric_map['a']

0

>>> value_to_numeric_map['b']

1

1.30 最大和最小的几个列表元素

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>>> a =[random.randint(0,100)for__ inxrange(100)]

>>> heapq.nsmallest(5, a)

[3,3,5,6,8]

>>> heapq.nlargest(5, a)

[100,100,99,98,98]

1.31 两个列表的笛卡尔积

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>>>forp initertools.product([1,2,3], [4,5]):

(1,4)

(1,5)

(2,4)

(2,5)

(3,4)

(3,5)

>>>forp initertools.product([0,1], repeat=4):

...    print''.join(str(x)forx inp)

...

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

1010

1011

1100

1101

1110

1111

1.32 列表组合和列表元素替代组合

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>>>forc initertools.combinations([1,2,3,4,5],3):

...    print''.join(str(x)forx inc)

...

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>>>forc initertools.combinations_with_replacement([1,2,3],2):

...    print''.join(str(x)forx inc)

...

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1.33 列表元素排列组合

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>>>forp initertools.permutations([1,2,3,4]):

...    print''.join(str(x)forx inp)

...

1234

4132

4213

4231

4312

4321

1.34 可链接迭代器

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>>> a =[1,2,3,4]

>>>forp initertools.chain(itertools.combinations(a, 2), itertools.combinations(a, 3)):

...    printp

...

(1,2)

(1,3)

(1,4)

(2,3)

(2,4)

(3,4)

(1,2,3)

(1,2,4)

(1,3,4)

(2,3,4)

>>>forsubset initertools.chain.from_iterable(itertools.combinations(a, n) forn inrange(len(a)+1))

...    printsubset

...

()

(1,)

(2,)

(3,)

(4,)

(1,2)

(1,3)

(1,4)

(2,3)

(2,4)

(3,4)

(1,2,3)

(1,2,4)

(1,3,4)

(2,3,4)

(1,2,3,4)