30个有关Python的小技巧

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1.1拆箱

>>>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+1foriinrange(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

>>>a,b=1,2
>>>a,b=b,a
>>>a,b
(2,1)

1.3扩展拆箱（只兼容python3）

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

1.4负数索引

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

1.5切割列表

>>>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

>>>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

>>>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

>>>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

>>>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

>>>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

>>>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

>>>a=[1,2,3,4,5,6]
>>>zip(*([iter(a)]*2))
[(1,2),(3,4),(5,6)]

>>>group_adjacent=lambdaa,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=lambdaa,k:zip(*(a[i::k]foriinrange(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

>>>defn_grams(a,n):
...z=[iter(a[i:])foriinrange(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

>>>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

>>>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]

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

>>>a=[[[1,2],[3,4]],[[5,6],[7,8]]]
>>>[xforl1inaforl2inl1forxinl2]
[1,2,3,4,5,6,7,8]

>>>a=[1,2,[3,4],[[5,6],[7,8]]]
>>>flatten=lambdax:[yforlinxforyinflatten(l)]iftype(x)islistelse[x]
>>>flatten(a)
[1,2,3,4,5,6,7,8]

1.16生成器表达式

>>>g=(x**2forxinxrange(10))
>>>next(g)
0
>>>next(g)
1
>>>next(g)
4
>>>next(g)
9
>>>sum(x**3forxinxrange(10))
2025
>>>sum(x**3forxinxrange(10)ifx%3==1)
408

1.17字典推导

>>>m={x:x**2forxinrange(5)}
>>>m
{0:0,1:1,2:4,3:9,4:16}

>>>m={x:'A'+str(x)forxinrange(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

>>>m={'a':1,'b':2,'c':3,'d':4}
>>>m
{'d':4,'a':1,'b':2,'c':3}
>>>{v:kfork,vinm.items()}
{1:'a',2:'b',3:'c',4:'d'}

1.19命名元组

>>>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

>>>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

>>>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])