有哪些有用的Python技巧

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函数连续调用

def add(x):      class AddNum(int):          def __call__(self, x):             return AddNum(self.numerator + x)      return AddNum(x)  print add(2)(3)(5)  # 10  print add(2)(3)(4)(5)(6)(7)  # 27  # javascript 版  var add = function(x){      var addNum = function(x){          return add(addNum + x);      };      addNum.toString = function(){          return x;      }      return addNum;  } add(2)(3)(5)//10  add(2)(3)(4)(5)(6)(7)//27

默认值陷阱

>>> def evil(v=[]):  ...     v.append(1)  ...     print v ...  >>> evil()  [1]  >>> evil()  [1, 1]

读写csv文件

import csv  with open('data.csv', 'rb') as f:      reader = csv.reader(f)      for row in reader:          print row  # 向csv文件写入  import csv  with open( 'data.csv', 'wb') as f:      writer = csv.writer(f)      writer.writerow(['name', 'address', 'age'])  # 单行写入      data = [              ( 'xiaoming ','china','10'),              ( 'Lily', 'USA', '12')]       writer.writerows(data)  # 多行写入

数制转换

>>> int('1000', 2)  8  >>> int('A', 16)  10

格式化 json

echo'{"k": "v"}' | python-m json.tool

list 扁平化

list_ = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]  [k for i in list_ for k in i] #[1, 2, 3, 4, 5, 6, 7, 8, 9]  import numpy as np  print np.r_[[1, 2, 3], [4, 5, 6], [7, 8, 9]]  import itertools  print list(itertools.chain(*[[1, 2, 3], [4, 5, 6], [7, 8, 9]]))  sum(list_, [])  flatten = lambda x: [y for l in x for y in flatten(l)] if type(x) is list else [x]  flatten(list_)

list 合并

>>> a = [1, 3, 5, 7, 9]  >>> b = [2, 3, 4, 5, 6]  >>> c = [5, 6, 7, 8, 9]  >>> list(set().union(a, b, c))  [1, 2, 3, 4, 5, 6, 7, 8, 9]

出现次数最多的 2 个字母

from collections import Counter  c = Counter('hello world')  print(c.most_common(2)) #[('l', 3), ('o', 2)]

谨慎使用

eval("__import__('os').system('rm -rf /')", {})

置换矩阵

matrix = [[1, 2, 3],[4, 5, 6]] res = zip( *matrix )   # res = [(1, 4), (2, 5), (3, 6)]

列表推导

[item**2 for item in lst if item % 2]  map(lambda item: item ** 2, filter(lambda item: item % 2, lst))  >>> list(map(str, [1, 2, 3, 4, 5, 6, 7, 8, 9]))  ['1', '2', '3', '4', '5', '6', '7', '8', '9']

排列组合

>>> for p in itertools.permutations([1, 2, 3, 4]):  ...     print ''.join(str(x) for x in p)  ...  1234  1243  1324  1342  1423  1432  2134  2143  2314  2341  2413  2431  3124  3142  3214  3241  3412  3421  4123  4132  4213  4231  4312  4321  >>> for c in itertools.combinations([1, 2, 3, 4, 5], 3):  ...     print ''.join(str(x) for x in c)  ...  123  124  125  134  135  145  234  235  245  345  >>> for c in itertools.combinations_with_replacement([1, 2, 3], 2):  ...     print ''.join(str(x) for x in c)  ...  11  12  13  22  23  33  >>> for p in itertools.product([1, 2, 3], [4, 5]):  (1, 4)  (1, 5)  (2, 4)  (2, 5)  (3, 4)  (3, 5)

默认字典

>>> m = dict()  >>> m['a']  Traceback (most recent call last):    File "", line 1, in   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]'

反转字典

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

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