Snippets

参考：https://medium.com/better-programming/20-python-snippets-you-should-learn-today-8328e26ff124

序列定长分割

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In [16]: from six.moves import zip_longest

In [17]: for i in zip_longest(*[iter([1,2,3,4,5])]*2, fillvalue=0):
    ...:     print(i)
    ...:
(1, 2)
(3, 4)
(5, 0)

In [18]: for i in zip_longest(*[iter('abcdefg')]*2, fillvalue='*'):
    ...:     print(i)
    ...:
('a', 'b')
('c', 'd')
('e', 'f')
('g', '*')

字符串逆转

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In [1]: my_string = "ABCDE"

In [2]: my_string[::-1]
Out[2]: 'EDCBA'

标题类（首字母大写）

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In [3]: my_string = "aaa bbb ccc ddd"

In [4]: my_string.title()
Out[4]: 'Aaa Bbb Ccc Ddd'

n次序列

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In [6]: my_string = "abcd"

In [7]: my_string*3
Out[7]: 'abcdabcdabcd'

In [8]: my_list = [1,2,3,4]

In [9]: my_list*3
Out[9]: [1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4]

列表解析

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In [8]: my_list=[1,2,3,4,5]

In [9]: [x for x in my_list if x%2!=0]
Out[9]: [1, 3, 5]

In [10]: my_list = [1,2,3,4]

In [11]: new_list = [2*x for x in my_list]

In [12]: new_list
Out[12]: [2, 4, 6, 8]

交换值

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In [15]: a,b=1,2

In [16]: a,b = b,a

In [17]: a
Out[17]: 2

连接字符串

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In [18]: list_of_strings = ['My', 'name', 'is', 'Chaitanya', 'Baweja']

In [19]: ','.join(list_of_strings)
Out[19]: 'My,name,is,Chaitanya,Baweja'

列表计数

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In [23]: from collections import Counter

In [24]: my_list = ['a','a','b','b','b','c','d','d','d','d','d']

In [25]: count=Counter(my_list)

In [26]: count
Out[26]: Counter({'a': 2, 'b': 3, 'c': 1, 'd': 5})

In [27]: count.most_common(1)
Out[27]: [('d', 5)]

合并字典

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In [28]: dict_1 = {'apple': 9, 'banana': 6}

In [29]: dict_2 = {'banana': 4, 'orange': 8}

In [30]: combined_dict = {**dict_1, **dict_2}

In [31]: combined_dict
Out[31]: {'apple': 9, 'banana': 4, 'orange': 8}

列表扁平化

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In [1]: from iteration_utilities import deepflatten

In [2]: l = [[1,2,3],[4,[5],[6,7]],[8,[9,[10]]]]

In [3]: print(list(deepflatten(l, depth=3)))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

列表采样

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In [4]: import random

In [5]: my_list = ['a', 'b', 'c', 'd', 'e']

In [6]: num_samples = 2

In [7]: random.sample(my_list,num_samples)
Out[7]: ['c', 'a']

对象内存

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In [5]: import sys

In [6]: a=10

In [7]: sys.getsizeof(a)
Out[7]: 28

常见问题

集合运算

注：集合不支持索引访问

操作符号列表

含义python
差集－/difference()
交集＆/intersection
并集｜/union
不等于！＝
等于＝＝
属于in
不属于not in

init.py 作用

https://www.jianshu.com/p/73f7fbf75183

__init__.py 该文件的作用就是相当于把自身整个文件夹当作一个包来管理，每当有外部import的时候，就会自动执行里面的函数。

二维list赋值问题

参考: (https://blog.csdn.net/zzc15806/article/details/82629406)(https://blog.csdn.net/zzc15806/article/details/82629406)

问题：以 arr = [[0]*N]*N 形式初始化时，对二维数组对某个元素赋值时会发现整列都会被赋值
原因：因为 [[0]*N]*N表示的是N个指向 [0]*N 这个列表的引用，所以当你修改某一个值时，整个列表都会被改变。
解决：以arr=[[0]*N for _ in range(N)]赋值。

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