复合if语句更快,还是多个if语句？

Question

假设我们有两段代码:

if foo == True and bar == False and baz == True:
    do something

和

if foo == True:
    if bar == False:
        if baz == True:
            do something

哪个更快？

Answer 1

在我的机器上使用IPython

In [1]: foo = True
In [2]: bar = False
In [3]: baz = True

In [4]: %%timeit
   ...: if foo and not bar and baz:
   ...:     lambda: None
1000000 loops, best of 3: 265 ns per loop

In [5]: %%timeit
   ...: if foo:
   ...:   if not bar:
   ...:     if baz:
   ...:       lambda: None
1000000 loops, best of 3: 275 ns per loop

如果将它拆分,看起来有10ns的开销.如果10ns很重要,你应该使用另一种语言.

所以对于所有实际目的,不,没有区别.

如果我们看得更深一点,我们可以看到微小的差异来自哪里.

In [6]: def compound():
   ...:     if foo and not bar and baz:
   ...:         lambda: None
In [7]: def multiple():
   ....:     if foo:
   ....:         if not bar:
   ....:             if baz:
   ....:                 lambda: None

In [8]: import dis
In [9]: dis.dis(compound)
  2           0 LOAD_GLOBAL              0 (foo)
              3 POP_JUMP_IF_FALSE       29
              6 LOAD_GLOBAL              1 (bar)
              9 UNARY_NOT           
             10 POP_JUMP_IF_FALSE       29
             13 LOAD_GLOBAL              2 (baz)
             16 POP_JUMP_IF_FALSE       29

  3          19 LOAD_CONST               1 (<code object <lambda> at 0x101d953b0, file "<ipython-input-9-d057c552d038>", line 3>)
             22 MAKE_FUNCTION            0
             25 POP_TOP             
             26 JUMP_FORWARD             0 (to 29)
        >>   29 LOAD_CONST               0 (None)
             32 RETURN_VALUE   

这有13条指令

In [15]: dis.dis(g)
  2           0 LOAD_GLOBAL              0 (foo)
              3 POP_JUMP_IF_FALSE       34

  3           6 LOAD_GLOBAL              1 (bar)
              9 POP_JUMP_IF_TRUE        34

  4          12 LOAD_GLOBAL              2 (baz)
             15 POP_JUMP_IF_FALSE       31

  5          18 LOAD_CONST               1 (<code object <lambda> at 0x101dbb530, file "<ipython-input-10-32b41e5f6f2b>", line 5>)
             21 MAKE_FUNCTION            0
             24 POP_TOP             
             25 JUMP_ABSOLUTE           31
             28 JUMP_ABSOLUTE           34
        >>   31 JUMP_FORWARD             0 (to 34)
        >>   34 LOAD_CONST               0 (None)
             37 RETURN_VALUE  

这有14条指令.

我在我的系统上使用默认的IPython做了这个,目前这是2.7.5,但你可以使用这种技术来配置任何你想要的任何你想要运行的Python版本.