while (1) vs. while(True) -- 为什么有区别(在python 2字节码中)? [英] while (1) vs. while(True) -- Why is there a difference (in python 2 bytecode)?

问题描述

对这个关于 perl 中无限循环的问题很感兴趣:while(1) 对比for (;;) 有速度差异吗?，我决定在python中运行一个类似的比较.我预计编译器会为 while(True): pass 和 while(1): pass 生成相同的字节码，但实际上在 python2 中并非如此.7.

以下脚本:

导入文件def while_one():而 1:经过def while_true():为真:经过打印(同时 1")打印(" -  -  -  -  -  -  -  -  -  -  -  -  -  - ")dis.dis(while_one)打印(当真")打印(" -  -  -  -  -  -  -  -  -  -  -  -  -  - ")dis.dis(while_true)

产生以下结果:

while 1-----------------------------4 0 SETUP_LOOP 3(到 6)5 >>3 JUMP_ABSOLUTE 3>>6 LOAD_CONST 0(无)9 RETURN_VALUE当真-----------------------------8 0 SETUP_LOOP 12(到 15)>>3 LOAD_GLOBAL 0(真)6 JUMP_IF_FALSE 4(到 13)9 POP_TOP9 10 JUMP_ABSOLUTE 3>>13 POP_TOP14 POP_BLOCK>>15 LOAD_CONST 0 (无)18 RETURN_VALUE

使用 while True 显然更加复杂.这是为什么?

在其他上下文中，python 的行为就像 True 等于 1:

<预><代码>>>>真 == 1真的>>>真 + 真2

为什么while要区分两者?

我注意到 python3 确实使用相同的操作评估语句:

while 1-----------------------------4 0 SETUP_LOOP 3(到 6)5 >>3 JUMP_ABSOLUTE 3>>6 LOAD_CONST 0(无)9 RETURN_VALUE当真-----------------------------8 0 SETUP_LOOP 3(到 6)9 >>3 JUMP_ABSOLUTE 3>>6 LOAD_CONST 0(无)9 RETURN_VALUE

python3 中布尔值的计算方式是否有变化?

解决方案

在 Python 2.x 中，True 不是关键字，而只是一个 内置全局常量，在 bool 类型中定义为 1.因此解释器仍然需要加载 True 的内容.换句话说，True 是可重新赋值的:

Python 2.7 (r27:82508, Jul 3 2010, 21:12:11)[GCC 4.0.1 (Apple Inc. build 5493)] 在达尔文上输入帮助"、版权"、信用"或许可"以获取更多信息.>>>真 = 4>>>真的4

<小时>

在 Python 3.x 它真正成为关键字和实常数:

Python 3.1.2(r312:79147，2010 年 7 月 19 日，21:03:37)[GCC 4.2.1 (Apple Inc. build 5664)] 在达尔文上输入帮助"、版权"、信用"或许可"以获取更多信息.>>>真 = 4文件<stdin>"，第 1 行语法错误:分配给关键字

因此解释器可以用无限循环替换 while True: 循环.

Intrigued by this question about infinite loops in perl: while (1) Vs. for (;;) Is there a speed difference?, I decided to run a similar comparison in python. I expected that the compiler would generate the same byte code for while(True): pass and while(1): pass, but this is actually not the case in python2.7.

The following script:

import dis

def while_one():
    while 1:
        pass

def while_true():
    while True:
        pass

print("while 1")
print("----------------------------")
dis.dis(while_one)

print("while True")
print("----------------------------")
dis.dis(while_true)

produces the following results:

while 1
----------------------------
  4           0 SETUP_LOOP               3 (to 6)

  5     >>    3 JUMP_ABSOLUTE            3
        >>    6 LOAD_CONST               0 (None)
              9 RETURN_VALUE        
while True
----------------------------
  8           0 SETUP_LOOP              12 (to 15)
        >>    3 LOAD_GLOBAL              0 (True)
              6 JUMP_IF_FALSE            4 (to 13)
              9 POP_TOP             

  9          10 JUMP_ABSOLUTE            3
        >>   13 POP_TOP             
             14 POP_BLOCK           
        >>   15 LOAD_CONST               0 (None)
             18 RETURN_VALUE        

Using while True is noticeably more complicated. Why is this?

In other contexts, python acts as though True equals 1:

>>> True == 1
True

>>> True + True
2

Why does while distinguish the two?

I noticed that python3 does evaluate the statements using identical operations:

while 1
----------------------------
  4           0 SETUP_LOOP               3 (to 6) 

  5     >>    3 JUMP_ABSOLUTE            3 
        >>    6 LOAD_CONST               0 (None) 
              9 RETURN_VALUE         
while True
----------------------------
  8           0 SETUP_LOOP               3 (to 6) 

  9     >>    3 JUMP_ABSOLUTE            3 
        >>    6 LOAD_CONST               0 (None) 
              9 RETURN_VALUE         

Is there a change in python3 to the way booleans are evaluated?

解决方案

In Python 2.x, True is not a keyword, but just a built-in global constant that is defined to 1 in the bool type. Therefore the interpreter still has to load the contents of True. In other words, True is reassignable:

Python 2.7 (r27:82508, Jul  3 2010, 21:12:11) 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> True = 4
>>> True
4

---

In Python 3.x it truly becomes a keyword and a real constant:

Python 3.1.2 (r312:79147, Jul 19 2010, 21:03:37) 
[GCC 4.2.1 (Apple Inc. build 5664)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> True = 4
  File "<stdin>", line 1
SyntaxError: assignment to keyword

thus the interpreter can replace the while True: loop with an infinite loop.

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