Python: how to do lazy debug logging

Question

I have some python like this: ``` def foo(): logger = logging.getLogger() # do something here logger.debug('blah blah {}'.format(expensive_func())) foo() ``` where `expensive_func()` is a function that returns string and that it is expensive to execute. When developping, the log level is set to DEBUG, and `expensive_func()` get executed, the message get logged, everything is fine. The problem is that when I set the log level strictly greater than DEBUG, say WARNING, in production env, obviously the return value of `expensive_func()` won't get logged, but the expensive function itself will still be executed. My question is: how to prevent python from excutting the expensive function when the logging level is WARNING? I don't want to delete that debug line or add something like `if level > DEBUG: return` in the expensive function. Thanks. EDIT I visited Lazy logger message string evaluation just now, but not satisfied with it, mainly because: - It's some what ugly; - Even if I wrap the…

Accepted Answer

Look at this part of the documentation.

Update: Logging already supports lazy evaluation, but slightly differently to the way described in your comnment. For example, see the following script:

```
import logging

def expensive_func(*args):
    print('Expensive func called: %s' % (args,))
    return sum(args)

class DeferredMessage(object):
    def __init__(self, func, *args):
        self.func = func
        self.args = args

def __str__(self):
        return 'Message {0}'.format(self.func(*self.args))

if __name__ == '__main__':
    logging.basicConfig()
    logging.info(DeferredMessage(expensive_func, 1, 2))
    logging.warning(DeferredMessage(expensive_func, 3, 4))
    logging.error(DeferredMessage(expensive_func, 5, 6))
```

When the above script is run, it should print

```
Expensive func called: (3, 4)
WARNING:root:Message 7
Expensive func called: (5, 6)
ERROR:root:Message 11
```

which shows that a potentially expensive function is only called when necessary. The example above can, of course, be generalised to allow the format string to be passed to the `DeferredMessage`, and to use `kwargs`, and so on.

Python: how to do lazy debug logging

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