Python 中的列表推导式

列表推导式是一种 Python 构造，可减少生成新列表或过滤现有列表所需的代码行。列表推导式包含在方括号内，它由一个表达式、一个或多个 for 循环和一个用于过滤生成的列表的可选条件组成。

列表推导式使用以下语法：

new_list = [expression(item) for item in iterable if condition]

翻译翻译就是：

new_list = []
for item in iterable:
    if condition:
        new_list.append(expression(item))

简单用法

最原始的1到10

numbers = [x for x in range(1,11)]
print(numbers)
>>> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

乘个2

numbers = [2*x for x in range(1,11)]
print(numbers)
>>> [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

只要偶数

numbers = [x for x in range(1,11) if x%2==0]
print(numbers)
>>> [2, 4, 6, 8, 10]

来个等差数列

numbers = [2*x-1 for x in range(1,11)]
print(numbers)
>>> [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

来个二维数组

numbers = [[x, x**2] for x in range(2,7)]
print(numbers)
>>> [[2, 4], [3, 9], [4, 16], [5, 25], [6, 36]]

添加多个条件

只要一个范围内

numbers = [2*x for x in range(1,11) if x>2 and x<5]
print(numbers)
>>> [6, 8]

要添加 else 条件，我们必须重新排列列表理解元素的顺序。一个三元表达式的顺序
我们必须将条件移动到 for 关键字之前，以便在不满足 if 条件时返回一个不同于 2*x 的值。

# 如果 x 的值介于2和5之间，则列表推导式返回 2*x，否则返回 3*x。
numbers = [2*x if x > 2 and x < 5 else 3*x for x in range(10)]
print(numbers)
>>> [0, 3, 6, 6, 8, 15, 18, 21, 24, 27]

如果条件在多一点

numbers = []
for x in range(10):
    if x > 2 and x < 5:
        numbers.append(2*x)
    elif x <=2:
        numbers.append(3*x)
    else:
        numbers.append(4*x)
print(numbers)
>>> [0, 3, 6, 6, 8, 20, 24, 28, 32, 36]

numbers = [2*x if x > 2 and x < 5 else 3*x if x <=2 else 4*x for x in range(10)]
print(numbers)
>>> [0, 3, 6, 6, 8, 20, 24, 28, 32, 36]

多个列表使用

与 zip 函数一起使用的列表推导返回一个元组列表，其中第 n 个元组包含每个列表的第 n 个元素。

cities = ['Rome', 'Warsaw', 'London']
countries = ['Italy', 'Poland', 'United Kingdom']
abc = [(city, country) for city, country in zip(cities, countries)]
print(abc)
>>> [('Rome', 'Italy'), ('Warsaw', 'Poland'), ('London', 'United Kingdom')]

cities = ['Rome', 'Warsaw', 'London']
countries = ['Italy', 'Poland', 'United Kingdom']
languages = ['Italian', 'Polish', 'English']
abc = [(city, country, language) for city, country, language in zip(cities, countries, languages)]
print(abc)
>>> [('Rome', 'Italy', 'Italian'), ('Warsaw', 'Poland', 'Polish'), ('London', 'United Kingdom', 'English')]

下面演示了两种不同的加法。挨个加一遍，，对应加，，

a = [2, 3, 4, 5]
b = [12, 13, 14, 15]
abc = [x+y for x in a for y in b]
print(abc)
>>> [14, 15, 16, 17, 15, 16, 17, 18, 16, 17, 18, 19, 17, 18, 19, 20]

a = [2, 3, 4, 5]
b = [12, 13, 14, 15]
abc = [x+y for x,y in zip(a,b)]
print(abc)
>>> [14, 16, 18, 20]

三个也是一样的

a = [2, 3]
b = [12, 13]
c = [22, 23]
abc = [x+y+z for x in a for y in b for z in c]
print(abc)
>>> [36, 37, 37, 38, 37, 38, 38, 39]

a = [2, 3, 4, 5]
b = [12, 13, 14, 15]
c = [22, 23, 24, 25]
abc = [x+y+z for x,y,z in zip(a,b,c)]
print(abc)
>>> [36, 39, 42, 45]

替换 Reduce 和 Lambda

reduce 函数返回总和

from functools import reduce
numbers = [3, 6, 8, 23]
print(reduce(lambda a,b: a+b, numbers))
>>> 40

修改一下

print(sum([number for number in numbers]))
>>> 40

嵌套列表推导式

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for row in matrix:
    for item in row:
        item  *= 2
print(matrix)
>>> [[2, 4, 6], [8, 10, 12], [14, 16, 18]]

简单地来个转换

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
matrix = [[item*2 for item in row] for row in matrix]
print(matrix)
>>> [[2, 4, 6], [8, 10, 12], [14, 16, 18]]