Python 为什么'if x is None：pass'比'x is None'单独使用更快

发布于03月26日

Python 3.12中的计时结果(在不同机器上与Python 3.11和Python 3.13类似):

When x = None:
13.8 ns  x is None
10.1 ns  if x is None: pass

When x = True:
13.9 ns  x is None
11.1 ns  if x is None: pass

more怎么能花less个时间呢？

为什么if x is None: pass更快，当它必须做相同的x is None判断，然后额外判断结果的真值(并做或跳过pass)？

其他版本/计算机上的时间:

Python3.11:(12.4，9.3)和(12.0，8.8)
Python3.13:(29.1，24.6)和(29.1，22.6)

基准脚本(Attempt This Online!):

from timeit import repeat
import sys

for x in None, True:
    print(f'When {x = }:')
    for code in ['x is None', 'if x is None: pass'] * 2:
        t = min(repeat(code, f'{x=}', repeat=100))
        print(f'{t*1e3:4.1f} ns ', code)
    print()

print('Python:', sys.version)

推荐答案

查看反汇编代码:

>>> import dis
>>> dis.dis('if x is None: pass')
  0           0 RESUME                   0

  1           2 LOAD_NAME                0 (x)
              4 POP_JUMP_IF_NOT_NONE     1 (to 8)
              6 RETURN_CONST             0 (None)
        >>    8 RETURN_CONST             0 (None)
>>> dis.dis('x is None')
  0           0 RESUME                   0

  1           2 LOAD_NAME                0 (x)
              4 LOAD_CONST               0 (None)
              6 IS_OP                    0
              8 RETURN_VALUE

if有一个特殊的POP_JUMP_IF_NOT_NONE运算，比LOAD_CONST加IS_OP快.你可以在这里阅读关于它的详细讨论:https://github.com/faster-cpython/ideas/discussions/154.