Best way to override lineno in Python logger

Question

I have written a decorator which logs the arguments used to call a particular function or method. As shown below, it works well except that the line number reported in the logRecord is the line number of the decorator rather than the line number of func that is being wrapped:

from functools import wraps
import inspect
import logging

arg_log_fmt = "{name}({arg_str})"


def log_args(logger, level=logging.DEBUG):
    """Decorator to log arguments passed to func."""
    def inner_func(func):
        line_no = inspect.getsourcelines(func)[-1]

        @wraps(func)
        def return_func(*args, **kwargs):
            arg_list = list("{!r}".format(arg) for arg in args)
            arg_list.extend("{}={!r}".format(key, val)
                            for key, val in kwargs.iteritems())
            msg = arg_log_fmt.format(name=func.__name__,
                                     arg_str=", ".join(arg_list))
            logger.log(level, msg)
            return func(*args, **kwargs)
        return return_func

    return inner_func

if __name__ == "__main__":
    logger = logging.getLogger(__name__)
    handler = logging.StreamHandler()
    fmt = "%(asctime)s %(levelname)-8.8s [%(name)s:%(lineno)4s] %(message)s"
    handler.setFormatter(logging.Formatter(fmt))
    logger.addHandler(handler)
    logger.setLevel(logging.DEBUG)


    @log_args(logger)
    def foo(x, y, z):
        pass

    class Bar(object):
        @log_args(logger)
        def baz(self, a, b, c):
            pass

    foo(1, 2, z=3)
    foo(1, 2, 3)
    foo(x=1, y=2, z=3)

    bar = Bar()
    bar.baz(1, c=3, b=2)

This sample results in the following output

2015-09-07 12:42:47,779 DEBUG    [__main__:  25] foo(1, 2, z=3)
2015-09-07 12:42:47,779 DEBUG    [__main__:  25] foo(1, 2, 3)
2015-09-07 12:42:47,779 DEBUG    [__main__:  25] foo(y=2, x=1, z=3)
2015-09-07 12:42:47,779 DEBUG    [__main__:  25] baz(<__main__.Bar object at 0x1029094d0>, 1, c=3, b=2)

Note that the line numbers all point to the decorator.

With inspect.getsourcelines(func) I can get the line number I'm interested in, but an attempt to over-write lineno in logger.debug results in an error. What is the best approach to getting the line number of the wrapped function to appear in the logging statement?

Answer 1

As Martijn points out, things sometimes change. However, since you're using Python 2 (the iteritems gave it away), the following code will work if you don't mind monkey-patching logging:

from functools import wraps
import logging

class ArgLogger(object):
    """
    Singleton class -- will only be instantiated once
    because of the monkey-patching of logger.
    """

    singleton = None

    def __new__(cls):
        self = cls.singleton
        if self is not None:
            return self
        self = cls.singleton = super(ArgLogger, cls).__new__(cls)
        self.code_location = None

        # Do the monkey patch exactly one time
        def findCaller(log_self):
            self.code_location, code_location = None, self.code_location
            if code_location is not None:
                return code_location
            return old_findCaller(log_self)
        old_findCaller = logging.Logger.findCaller
        logging.Logger.findCaller = findCaller

        return self

    def log_args(self, logger, level=logging.DEBUG):
        """Decorator to log arguments passed to func."""
        def inner_func(func):
            co = func.__code__
            code_loc = (co.co_filename, co.co_firstlineno, co.co_name)

            @wraps(func)
            def return_func(*args, **kwargs):
                arg_list = list("{!r}".format(arg) for arg in args)
                arg_list.extend("{}={!r}".format(key, val)
                                for key, val in kwargs.iteritems())
                msg = "{name}({arg_str})".format(name=func.__name__,
                                        arg_str=", ".join(arg_list))
                self.code_location = code_loc
                logger.log(level, msg)
                return func(*args, **kwargs)
            return return_func

        return inner_func


log_args = ArgLogger().log_args

if __name__ == "__main__":
    logger = logging.getLogger(__name__)
    handler = logging.StreamHandler()
    fmt = "%(asctime)s %(levelname)-8.8s [%(name)s:%(lineno)4s] %(message)s"
    handler.setFormatter(logging.Formatter(fmt))
    logger.addHandler(handler)
    logger.setLevel(logging.DEBUG)


    @log_args(logger)
    def foo(x, y, z):
        pass

    class Bar(object):
        @log_args(logger)
        def baz(self, a, b, c):
            pass

    def test_regular_log():
        logger.debug("Logging without ArgLog still works fine")

    foo(1, 2, z=3)
    foo(1, 2, 3)
    foo(x=1, y=2, z=3)

    bar = Bar()
    bar.baz(1, c=3, b=2)
    test_regular_log()

Answer 2

You can't easily change the line number, because the Logger.findCaller() method extracts this information via introspection.

You could re-build the function and code objects for the wrapper function you generate, but that is very hairy indeed (see the hoops I and Veedrac jump through on this post) and will lead to problems when you have a bug, as your traceback will show the wrong source lines!

You'd be better off adding the line number, as well as your module name (since that can differ too) to your log output manually:

arg_log_fmt = "{name}({arg_str}) in {filename}:{lineno}"

# ...

codeobj = func.__code__
msg = arg_log_fmt.format(
    name=func.__name__, arg_str=", ".join(arg_list),
    filename=codeobj.co_filename, lineno=codeobj.co_firstlineno)

Since you always have a function here, I used some more direct introspection to get the first line number for the function, via the associated code object.

Answer 3

Another possibility is to subclass Logger to override Logger.makeRecord. This is the method that raises a KeyError if you try to change any of the standard attributes (like rv.lineno) in the LogRecord:

for key in extra:
    if (key in ["message", "asctime"]) or (key in rv.__dict__):
        raise KeyError("Attempt to overwrite %r in LogRecord" % key)
    rv.__dict__[key] = extra[key]

By removing this precaution, we can override the lineno value by supplying an extra argument to the logger.log call:

logger.log(level, msg, extra=dict(lineno=line_no))

---

from functools import wraps
import inspect
import logging

arg_log_fmt = "{name}({arg_str})"


def makeRecord(self, name, level, fn, lno, msg, args, exc_info, func=None, extra=None):
    """
    A factory method which can be overridden in subclasses to create
    specialized LogRecords.
    """
    rv = logging.LogRecord(name, level, fn, lno, msg, args, exc_info, func)
    if extra is not None:
        rv.__dict__.update(extra)
    return rv

def log_args(logger, level=logging.DEBUG, cache=dict()):
    """Decorator to log arguments passed to func."""
    logger_class = logger.__class__
    if logger_class in cache:
        UpdateableLogger = cache[logger_class]
    else:
        cache[logger_class] = UpdateableLogger = type(
            'UpdateableLogger', (logger_class,), dict(makeRecord=makeRecord))

    def inner_func(func):
        line_no = inspect.getsourcelines(func)[-1]
        @wraps(func)
        def return_func(*args, **kwargs):
            arg_list = list("{!r}".format(arg) for arg in args)
            arg_list.extend("{}={!r}".format(key, val)
                            for key, val in kwargs.iteritems())
            msg = arg_log_fmt.format(name=func.__name__,
                                     arg_str=", ".join(arg_list))
            logger.__class__ = UpdateableLogger
            try:
                logger.log(level, msg, extra=dict(lineno=line_no))
            finally:
                logger.__class__ = logger_class
            return func(*args, **kwargs)
        return return_func

    return inner_func

if __name__ == "__main__":
    logger = logging.getLogger(__name__)
    handler = logging.StreamHandler()
    fmt = "%(asctime)s %(levelname)-8.8s [%(name)s:%(lineno)4s] %(message)s"
    handler.setFormatter(logging.Formatter(fmt))
    logger.addHandler(handler)
    logger.setLevel(logging.DEBUG)

    @log_args(logger)
    def foo(x, y, z):
        pass

    class Bar(object):

        @log_args(logger)
        def baz(self, a, b, c):
            pass

    foo(1, 2, z=3)
    foo(1, 2, 3)
    foo(x=1, y=2, z=3)

    bar = Bar()
    bar.baz(1, c=3, b=2)

yields

2015-09-07 16:01:22,332 DEBUG    [__main__:  48] foo(1, 2, z=3)
2015-09-07 16:01:22,332 DEBUG    [__main__:  48] foo(1, 2, 3)
2015-09-07 16:01:22,332 DEBUG    [__main__:  48] foo(y=2, x=1, z=3)
2015-09-07 16:01:22,332 DEBUG    [__main__:  53] baz(<__main__.Bar object at 0x7f17f75b0490>, 1, c=3, b=2)

---

The line

    UpdateableLogger = type('UpdateableLogger', (type(logger),), 
                            dict(makeRecord=makeRecord))

creates a new class which is a subclass of type(logger) which overrides makeRecord. Inside return_func, the logger's class is changed to UpdateableLogger so the call to logger.log can modify lineno and then the original logger class is restored.

By doing it this way -- by avoiding monkey-patching Logger.makeRecord -- all loggers behave exactly as before outside the decorated functions.

---

For comparison, the monkey-patching approach is shown here.