体验内存视图

python

体验内存视图

https://stackoverflow.com/questions/18655648/what-exactly-is-the-point-of-memoryview-in-python

memoryview是一个内置类，可以实现在不复制内容的条件下高效操作同一个数组的不同切片

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import time
for n in (100000, 200000, 300000, 400000):
    data = b'x'*n
    b = data
    start = time.time()
    while b:
        b = b[1:]
    end = time.time()
    print(end-start)
0.1373758316040039
0.5009992122650146
1.2156774997711182
2.221696615219116
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for n in (100000, 200000, 300000, 400000):
    data = b'x'*n
    start = time.time()
    b = memoryview(data)
    while b:
        b = b[1:]
    end = time.time()
    print(end-start)
0.01299595832824707
0.024030685424804688
0.03499794006347656
0.04696965217590332

一些细节

一个python对象的布尔值取决于它的类中__bool__方法和__len__方法的实现方式. 如果类实现了__bool__方法，则这个类的实例的布尔值就是__bool__方法的返回值. 如果__bool__方法没有实现，则取决于__len__方法的返回值.

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class Toy1:
    def __bool__(self):
        return True
    def __len__(self):
        return False
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if Toy1():
    print('chichi')
chichi
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class Toy2:
    def __len__(self):
        return False
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if not Toy2():
    print('chichi')
chichi