Python中有条件评估的调试语句 [英] Conditionally evaluated debug statements in Python

问题描述

Python有几种打印跟踪"输出的方法. print ， import logging ， stdout.write 可以用于打印调试信息，但是它们都有一个缺点:即使记录器的阈值是太高或流被关闭，Python仍将评估print语句的参数.(严格评估)这可能会花费字符串格式或更多的字符串格式.

Python has a few ways of printing "trace" output. print, import logging, stdout.write can be used to print debugging info, but they all have one drawback: even if the logger's threshold is too high or the stream is closed, Python will still evaluate the arguments to the print statement. (Strict Evaluation) This could cost a string format or more.

最明显的解决方法是将创建字符串的代码放入lambda，并使用我们自己的日志记录功能有条件地调用lambda(此代码检查 __ debug __ 内置变量，该变量设置为False每当python以 -O 启动以进行优化时):

The obvious fix is to put the string-creating code into a lambda, and use our own logging function to call the lambda conditionally (this one checks the __debug__ builtin variable, which is set to False whenever python is started with -O for optimizations) :

def debug(f):
  if __debug__:
    print f()
    #stdout.write(f())
    #logging.debug(f())

for currentItem in allItems:
  debug(lambda:"Working on {0}".format(currentItem))

优点是不会在发行版中调用 str(currentItem)和 string.format ，缺点是必须输入 lambda:在每个日志记录语句上.

The advantage is not calling str(currentItem) and string.format in release builds, and the disadvantage is having to type in lambda: on every logging statement.

Python的 assert 语句.如果python是用 -O 运行的，那么所有assert语句都将被丢弃，而不会进行任何评估.您可以利用它来编写另一个条件评估的日志记录语句:

Python's assert statement is treated specially by the Python compiler. If python is run with -O, then any assert statements are discarded without any evaluation. You can exploit this to make another conditionally-evaluated logging statement:

assert(logging.debug("Working on {0}".format(currentItem)) or True)

使用 -O 启动Python时，不会评估此行.

This line will not be evaluated when Python is started with -O.

甚至可以使用短路运算符'and'和'or':

The short-circuit operators 'and' and 'or' can even be used:

__debug__ and logging.debug("Working on {0}".format(currentItem));

但是现在我们最多可以包含28个字符以及输出字符串的代码.

But now we're up to 28 characters plus the code for the output string.

我要解决的问题:是否有任何标准的python语句或函数具有与 assert 语句相同的条件评估属性?或者，有人可以替代这里介绍的方法吗?

The question I'm getting to: Are there any standard python statements or functions that have the same conditional-evaluation properties as the assert statement? Or, does anyone have any alternatives to the methods presented here?

推荐答案

我不知道在没有处理程序的情况下，对 logging.debug 的调用会影响性能多少.

I wonder how much a call to logging.debug impacts the performance when there are no handlers.

但是 if __debug __:语句仅被评估一次，即使在函数体内也是如此

However the if __debug__: statement is evaluated only once, even in the body of a function

$ python -O
Python 2.6.6 (r266:84292, Dec 26 2010, 22:31:48)
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import dis
>>> import logging
>>> def debug(*a, **kw):
...  if __debug__:
...   logging.debug(*a, **kw)
... 
>>> dis.dis(debug)
  2           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE        
>>> 

，记录器可以使用字符串格式运算符为您格式化消息.这是从 logging.debug文档

and the logger can format the message for you using the string formatting operator. Here a slightly modified example taken from the logging.debug documentation

FORMAT = '%(asctime)-15s %(clientip)s %(user)-8s %(message)s'
logging.basicConfig(format=FORMAT)
d = { 'clientip' : '192.168.0.1', 'user' : 'fbloggs' }
debug('Protocol problem: %s', 'connection reset', extra=d)

在这种情况下，如果关闭优化，则永远不会评估消息字符串.

In this case the message string is never evaluated if the optimizations are turned off.

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