你如何实现“#ifdef"?在蟒蛇? [英] How do you implement "#ifdef" in python?

问题描述

C 中的编程 我曾经有一些代码段仅用于调试目的(记录命令等).通过使用 #ifdef 预处理器指令，可以完全禁用这些语句，如下所示:

 #ifdef 宏受控文本#endif/* 宏 */

在 python 中执行类似操作的最佳方法是什么?

解决方案

如果您只想禁用日志记录方法，请使用 logging 模块.如果日志级别设置为排除，比如调试语句，那么 logging.debug 将非常接近无操作(它只检查日志级别并返回而不插入日志字符串).

如果您想在以特定变量为条件的字节码编译时实际删除代码块，您唯一的选择是相当神秘的 __debug__ 全局变量.除非将 -O 标志传递给 Python(或将 PYTHONOPTIMIZE 设置为环境中的非空值)，否则此变量设置为 True.

如果在if语句中使用__debug__，if语句实际上只编译成True分支.这种特殊的优化与 Python 曾经获得的预处理器宏一样接近.

请注意，与宏不同，您的代码在 if 的两个分支中的语法仍然必须正确.

<小时>

要展示 __debug__ 的工作原理，请考虑以下两个函数:

def f():如果 __debug__:返回 3否则:返回 4定义 g():如果为真:返回 3否则:返回 4

现在用dis检查它们:

<预><代码>>>>dis.dis(f)2 0 LOAD_CONST 1 (3)3 RETURN_VALUE>>>dis.dis(g)2 0 LOAD_GLOBAL 0(真)3 JUMP_IF_FALSE 5(到 11)6 POP_TOP7 LOAD_CONST 1 (3)10 RETURN_VALUE>>11 POP_TOP3 12 LOAD_CONST 2 (4)15 RETURN_VALUE16 LOAD_CONST 0 (无)19 RETURN_VALUE

如您所见，只有 f 是优化的".

Programming in C I used to have code sections only used for debugging purposes (logging commands and the like). Those statements could be completely disabled for production by using #ifdef pre-processor directives, like this:

 #ifdef MACRO

 controlled text

 #endif /* MACRO */

What is the best way to do something similar in python?

解决方案

If you just want to disable logging methods, use the logging module. If the log level is set to exclude, say, debug statements, then logging.debug will be very close to a no-op (it just checks the log level and returns without interpolating the log string).

If you want to actually remove chunks of code at bytecode compile time conditional on a particular variable, your only option is the rather enigmatic __debug__ global variable. This variable is set to True unless the -O flag is passed to Python (or PYTHONOPTIMIZE is set to something nonempty in the environment).

If __debug__ is used in an if statement, the if statement is actually compiled into only the True branch. This particular optimization is as close to a preprocessor macro as Python ever gets.

Note that, unlike macros, your code must still be syntactically correct in both branches of the if.

---

To show how __debug__ works, consider these two functions:

def f():
    if __debug__: return 3
    else: return 4

def g():
    if True: return 3
    else: return 4

Now check them out with dis:

>>> dis.dis(f)
  2           0 LOAD_CONST               1 (3)
              3 RETURN_VALUE        
>>> dis.dis(g)
  2           0 LOAD_GLOBAL              0 (True)
              3 JUMP_IF_FALSE            5 (to 11)
              6 POP_TOP             
              7 LOAD_CONST               1 (3)
             10 RETURN_VALUE        
        >>   11 POP_TOP             

  3          12 LOAD_CONST               2 (4)
             15 RETURN_VALUE        
             16 LOAD_CONST               0 (None)
             19 RETURN_VALUE        

As you can see, only f is "optimized".

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