python列表生成式

python列表生成式是一种基于其他iterable（如集合、元组、其他列表等）创建列表的方法。这种方法将整个输出列表加载到内存中，对于中小型的列表，这个是可取的，它可以使操作更快。大型的列表就不合适，不可取的，由于内存需求过大可能导致计算机崩溃。

下面介绍一下列表生成式：

import time
new_squares = [value**2 for value in range(1,51)]
print(new_squares)
print(time.thread_time())

输出结果为：

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841, 900, 961, 1024, 1089, 1156, 1225, 1296, 1369, 1444, 1521, 1600, 1681, 1764, 1849, 1936, 2025, 2116, 2209, 2304, 2401, 2500]
0.8125

普通的迭代：

import time

squares = []
for value in range(1,51):
    squares.append(value**2)
print(squares)
print(time.thread_time())

输出结果为：

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841, 900, 961, 1024, 1089, 1156, 1225, 1296, 1369, 1444, 1521, 1600, 1681, 1764, 1849, 1936, 2025, 2116, 2209, 2304, 2401, 2500]
0.84375

紧接着我们来测试两个代码运行时占用的内存大小，我们引入一个psutil包，进行测试

"""set_out_list,py"""
import os
import psutil

def count_info(count):
    def float_info():
        pid = os.getpid()
        p = psutil.Process(pid)
        info_start = p.memory_full_info().uss/1024
        count()
        info_end = p.memory_full_info().uss/1024
        print("占用"+str(info_end-info_start)+"kb")
    return float_info

@count_info
def main():
    squares = []
    for value in range(1,51):
        if value%2 == 0:
            squares.append(value**2)
    print(squares)

if __name__ == '__main__':
    main()

"""set_out_list_new.py"""
import os
import psutil

def count_info(count):
    def float_info():
        pid = os.getpid()
        p = psutil.Process(pid)
        info_start = p.memory_full_info().uss/1024
        count()
        info_end = p.memory_full_info().uss/1024
        print("占用"+str(info_end-info_start)+"kb")
    return float_info

@count_info
def main():
    new_squares = [value**2 for value in range(1,51) if value%2==0]
    print(new_squares)

if __name__ == '__main__':
    main()

运行结果图：

PS E:pycharmPyCharm 2021.3pythonproject> python .set_out_list.py
[4, 16, 36, 64, 100, 144, 196, 256, 324, 400, 484, 576, 676, 784, 900, 1024, 1156, 1296, 1444, 1600, 1764, 1936, 2116, 2304, 2500]
占用4.0kb
PS E:pycharmPyCharm 2021.3pythonproject> python .set_out_list_new.py
[4, 16, 36, 64, 100, 144, 196, 256, 324, 400, 484, 576, 676, 784, 900, 1024, 1156, 1296, 1444, 1600, 1764, 1936, 2116, 2304, 2500]
占用4.0kb

我们发现内存占用一样都是4.0kb。