为什么 `{*l}` 比 `set(l)` 快 - python 集合(不仅适用于集合，适用于所有序列)

2022-01-17 00:00:00 python set sequence timing unpack

问题描述

所以这是我的时间安排:

So here is my timings:

>>> import timeit
>>> timeit.timeit(lambda: set(l))
0.7210583936611334
>>> timeit.timeit(lambda: {*l})
0.5386332845236943

为什么会这样，我的意见是平等的，但事实并非如此.

Why is that, my opinion would be equal but it's not.

所以从这个例子中解包很快，对吧?

So unpacking is fast from this example, right?

解决方案

同理[]比 list() 快；解释器包括对使用专门代码路径的基于语法的操作的专门支持，而构造函数调用涉及:

For the same reason [] is faster than list(); the interpreter includes dedicated support for syntax based operations that uses specialized code paths, while constructor calls involve:

  1. 从内置范围加载构​​造函数(需要一对 dict 查找，一个在全局范围内，另一个在内置范围内失败时)
  2. 需要通过通用可调用调度机制和通用参数解析代码进行调度，所有这些都比将其所有参数作为 C 数组从堆栈中读取的单字节代码昂贵得多
  1. Loading the constructor from built-in scope (requires a pair of dict lookups, one in global scope, then another in built-in scope when it fails)
  2. Requires dispatch through generic callable dispatch mechanisms, and generic argument parsing code, all of which is far more expensive than a single byte code that reads all of its arguments off the stack as a C array

所有这些优点都与固定开销有关；两种方法的 big-O 相同，因此 {*range(10000)} 不会明显/可靠地比 set(range(10000)) 快，因为实际的构造工作远远超过了通过泛型调度加载和调用构造函数的开销.

All of these advantages relate to fixed overhead; the big-O of both approaches are the same, so {*range(10000)} won't be noticeably/reliably faster than set(range(10000)), because the actual construction work vastly outweighs the overhead of loading and calling the constructor via generic dispatch.

相关文章