Python列表生成式

python里面[]表示一个列表，对容器类型的数据进行运算和操作，生成新的列表最高效、快速的办法，就是列表生成式。它优雅、简洁，值得大家多多使用！今天盘点列表生成式在工作中的主要使用场景。

1.入门例子

range快速生成连续列表

In [1]: a = range(11)

In [2]: a
Out[2]: range(0, 11)

In [3]: list(a)
Out[3]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

对列表里面的数据进行运算后重新生成一个新的列表：

In [5]: a = range(0,11)

In [6]: b = [x**2 for x in a]

In [7]: b
Out[7]: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

对一个列表里面的数据筛选，只计算[0,11) 中偶数的平方：

In [10]: a = range(11)

In [11]: c = [x**2 for x in a if x%2==0]

In [12]: c
Out[12]: [0, 4, 16, 36, 64, 100]

前面列表生成式都只传一个参数x，带有两个参数的运算：

In [13]: a = range(5)

In [14]: b = ['a','b','c','d','e']
In [20]: c = [str(y) + str(x) for x, y in zip(a,b)]
In [21]: c
Out[21]: ['a0', 'b1', 'c2', 'd3', 'e4']

2.中级例子

结合字典，打印键值对：

In [22]: a = {'a':1,'b':2,'c':3}
In [23]: b = [k+ '=' + v for k, v in a.items()]
In [24]: b = [k+ '=' + str(v) for k, v in a.items()]
In [25]: b
Out[25]: ['a=1', 'b=2', 'c=3']

输出某个目录下的所有文件和文件夹的名称：

In [33]: [d for d in os.listdir('d:/summary')]
Out[33]: ['a.txt.txt', 'python-100']

列表中所有单词都转化为小写：

In [34]: a = ['Hello', 'World', '2019Python']

In [35]: [w.lower() for w in a]
Out[35]: ['hello', 'world', '2019python']

3.高级例子

将值分组：

In [36]: def bifurcate(lst, filter):
    ...:   return [
    ...:     [x for i,x in enumerate(lst) if filter[i] == True],
    ...:     [x for i,x in enumerate(lst) if filter[i] == False]
    ...:   ]
    ...:
In [37]: bifurcate(['beep', 'boop', 'foo', 'bar'], [True, True, False, True])

Out[37]: [['beep', 'boop', 'bar'], ['foo']]

进一步抽象上个例子，根据指定函数fn 对lst 分组：

In [38]: def bifurcate_by(lst, fn):
    ...:   return [
    ...:     [x for x in lst if fn(x)],
    ...:     [x for x in lst if not fn(x)]
    ...:   ]
    ...:

In [39]: bifurcate_by(['beep', 'boop', 'foo', 'bar'], lambda x: x[0] == 'b')
Out[39]: [['beep', 'boop', 'bar'], ['foo']]

返回可迭代对象的差集，注意首先都把a, b用set 包装

In [53]: def difference(a, b):
    ...:   _a, _b =set(a),set(b)
    ...:   return [item for item in _a if item not in _b]
    ...:
    ...:

In [54]: difference([1,1,2,3,3], [1, 2, 4])
Out[54]: [3]

进一步抽象上个例子，根据函数fn 映射后选取差集，如下列表元素分别为单个元素和字典的例子：

In [61]: def difference_by(a, b, fn):
    ...:     ...:   _b = set(map(fn, b))
    ...:     ...:   return [item for item in a if fn(item) not in _b]
    ...:     ...:
    ...:

In [62]: from math import floor
    ...: difference_by([2.1, 1.2], [2.3, 3.4],floor)
Out[62]: [1.2]

In [63]: difference_by([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], lambda v : v['x'])
Out[63]: [{'x': 2}]

过滤非重复值，结合list 的count( 统计出元素在列表中出现次数)：

In [64]: def filter_non_unique(lst):
    ...:   return [item for item in lst if lst.count(item) == 1]

In [65]: filter_non_unique([1, 2, 2, 3, 4, 4, 5])
Out[65]: [1, 3, 5]