作者 | zglg

來源 | Python與算法社區(qū)

python里[] 表示一個(gè)列表，對容器類型的數(shù)據(jù)進(jìn)行運(yùn)算和操作，生成新的列表最高效、快速的辦法，就是列表生成式。

它優(yōu)雅、簡潔，值得大家多多使用！今天盤點(diǎn)列表生成式在工作中的主要使用場景。

入門

1

range快速生成連續(xù)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]

2

對列表里面的數(shù)據(jù)進(jìn)行運(yùn)算后重新生成一個(gè)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]

3

對一個(gè)列表里面的數(shù)據(jù)篩選，只計(jì)算[0,11) 中偶數(shù)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]

4

前面列表生成式都只傳一個(gè)參數(shù)x，帶有兩個(gè)參數(shù)的運(yùn)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']

5

結(jié)合字典，打印鍵值對：

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']

6

輸出某個(gè)目錄下的所有文件和文件夾的名稱：

In [33]: [d for d in os.listdir('d:/summary')]

Out[33]: ['a.txt.txt', 'python-100']

7

列表中所有單詞都轉(zhuǎn)化為小寫：

In [34]: a = ['Hello', 'World', '2019Python']
In [35]: [w.lower() for w in a]
Out[35]: ['hello', 'world', '2019python']

進(jìn)階

8

將值分組：

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']]

9

進(jìn)一步抽象例子8，根據(jù)指定函數(shù)fn 對lst 分組：

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']]

10

返回可迭代對象的差集，注意首先都把a(bǔ), b用set 包裝

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]

11

進(jìn)一步抽象10，根據(jù)函數(shù)fn 映射后選取差集，如下列表元素分別為單個(gè)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}]

12

過濾非重復(fù)值，結(jié)合list 的count( 統(tǒng)計(jì)出元素在列表中出現(xiàn)次數(shù))：

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]

熟練操作以上12個(gè)例子，就算掌握python 中非常有用的列表生成式。