Logging uncaught exceptions in Python

Question

Here's a complete small example that also includes a few other tricks:

import sys
import logging
logger = logging.getLogger(__name__)
handler = logging.StreamHandler(stream=sys.stdout)
logger.addHandler(handler)

def handle_exception(exc_type, exc_value, exc_traceback):
    if issubclass(exc_type, KeyboardInterrupt):
        sys.__excepthook__(exc_type, exc_value, exc_traceback)
        return

    logger.error("Uncaught exception", exc_info=(exc_type, exc_value, exc_traceback))

sys.excepthook = handle_exception

if __name__ == "__main__":
    raise RuntimeError("Test unhandled")

• Ignore KeyboardInterrupt so a console python program can exit with Ctrl + C.

• Rely entirely on python's logging module for formatting the exception.

• Use a custom logger with an example handler. This one changes the unhandled exception to go to stdout rather than stderr, but you could add all sorts of handlers in this same style to the logger object.

As Ned pointed out, sys.excepthook is invoked every time an exception is raised and uncaught. The practical implication of this is that in your code you can override the default behavior of sys.excepthook to do whatever you want (including using logging.exception).

As a straw man example:

import sys
def foo(exctype, value, tb):
    print('My Error Information')
    print('Type:', exctype)
    print('Value:', value)
    print('Traceback:', tb)

Override sys.excepthook:

>>> sys.excepthook = foo

Commit obvious syntax error (leave out the colon) and get back custom error information:

>>> def bar(a, b)
My Error Information
Type: <type 'exceptions.SyntaxError'>
Value: invalid syntax (<stdin>, line 1)
Traceback: None

For more information about sys.excepthook, read the docs.

Why not:

import sys
import logging
import traceback

def log_except_hook(*exc_info):
    text = "".join(traceback.format_exception(*exc_info()))
    logging.error("Unhandled exception: %s", text)

sys.excepthook = log_except_hook

None()

Here is the output with sys.excepthook as seen above:

$ python tb.py
ERROR:root:Unhandled exception: Traceback (most recent call last):
  File "tb.py", line 11, in <module>
    None()
TypeError: 'NoneType' object is not callable

Here is the output with the sys.excepthook commented out:

$ python tb.py
Traceback (most recent call last):
  File "tb.py", line 11, in <module>
    None()
TypeError: 'NoneType' object is not callable

The only difference is that the former has ERROR:root:Unhandled exception: at the beginning of the first line.

The method sys.excepthook will be invoked if an exception is uncaught: http://docs.python.org/library/sys.html#sys.excepthook

When an exception is raised and uncaught, the interpreter calls sys.excepthook with three arguments, the exception class, exception instance, and a traceback object. In an interactive session this happens just before control is returned to the prompt; in a Python program this happens just before the program exits. The handling of such top-level exceptions can be customized by assigning another three-argument function to sys.excepthook.

To build on Jacinda's answer, but using a logger object:

def catchException(logger, typ, value, traceback):
    logger.critical("My Error Information")
    logger.critical("Type: %s" % typ)
    logger.critical("Value: %s" % value)
    logger.critical("Traceback: %s" % traceback)

# Use a partially applied function
func = lambda typ, value, traceback: catchException(logger, typ, value, traceback)
sys.excepthook = func

In my case (using python 3) when using @Jacinda 's answer the content of the traceback was not printed. Instead, it just prints the object itself: <traceback object at 0x7f90299b7b90>.

Instead I do:

import sys
import logging
import traceback

def custom_excepthook(exc_type, exc_value, exc_traceback):
    # Do not print exception when user cancels the program
    if issubclass(exc_type, KeyboardInterrupt):
        sys.__excepthook__(exc_type, exc_value, exc_traceback)
        return

    logging.error("An uncaught exception occurred:")
    logging.error("Type: %s", exc_type)
    logging.error("Value: %s", exc_value)

    if exc_traceback:
        format_exception = traceback.format_tb(exc_traceback)
        for line in format_exception:
            logging.error(repr(line))

sys.excepthook = custom_excepthook

Wrap your app entry call in a try...except block so you'll be able to catch and log (and perhaps re-raise) all uncaught exceptions. E.g. instead of:

if __name__ == '__main__':
    main()

Do this:

if __name__ == '__main__':
    try:
        main()
    except Exception as e:
        logger.exception(e)
        raise