使用修饰函数作为函数的默认参数

Question

Consider this module:

#mymodule.py
import logging

def print_start_end(name):
    """
    Decorator that creates a logger and logs the start 
    and the end of the function call
    """
    def decorator(f):
        logger = logging.getLogger(name)
        def wrapper(*args, **kwargs):
            logger.info("start")
            res = f(*args, **kwargs)
            logger.info("end")
            return res
        return wrapper
    return decorator

@print_start_end(__name__)
def f(x):
    return x

def g(y=f(3)):
    return y

And an example script:

import logging
from mymodule import f

logger = logging.getLogger("")
logger.setLevel(logging.INFO)
h = logging.StreamHandler()
h.setLevel(logging.INFO)
logger.addHandler(h)

print f(3)

Output:

start
end
3

The decorator is working. But now I write a script to use g instead of f :

import logging
from mymodule import g

logger = logging.getLogger("")
logger.setLevel(logging.INFO)
h = logging.StreamHandler()
h.setLevel(logging.INFO)
logger.addHandler(h)

print g()

Output:

3

When executing the statement print g() , the call to f was successful because it printed the expected return value 3. But why doesn't it also print "start" and "end"?

Answer 1

The problem is that the default value is evaluated immediately, before the logger has been initialized. The way around this is to defer execution until afterwards.

def g(y=lambda:f(3)):
    return y()

Answer 2

Note that the f(3) in def g(y=f(3)): is executed only once , when the function is defined, not every time when it is called.

Thus, the problem seems to be that by the time f(3) is executed in mymodule , the logger is not yet initialized. Initialize first, then import, then it works:

import logging

logger = logging.getLogger("")
logger.setLevel(logging.INFO)
h = logging.StreamHandler()
h.setLevel(logging.INFO)
logger.addHandler(h)

from mymodule import g

print g()