Calling a hook function every time an Exception is raised

Let's say I want to be able to log to file every time any exception is raised, anywhere in my program. I don't want to modify any existing code.

Of course, this could be generalized to being able to insert a hook every time an exception is raised.

Would the following code be considered safe for doing such a thing?

class MyException(Exception):

    def my_hook(self):
        print('---> my_hook() was called');

    def __init__(self, *args, **kwargs):
        global BackupException;
        self.my_hook();
        return BackupException.__init__(self, *args, **kwargs);


def main():
    global BackupException;
    global Exception;

    BackupException = Exception;
    Exception = MyException;

    raise Exception('Contrived Exception');


if __name__ == '__main__':
    main();

Solution 1:

If you want to log uncaught exceptions, just use sys.excepthook.

I'm not sure I see the value of logging all raised exceptions, since lots of libraries will raise/catch exceptions internally for things you probably won't care about.

Solution 2:

Your code as far as I can tell would not work.

1. __init__ has to return None and you are trying to return an instance of backup exception. In general if you would like to change what instance is returned when instantiating a class you should override __new__.

2. Unfortunately you can't change any of the attributes on the Exception class. If that was an option you could have changed Exception.__new__ and placed your hook there.

3. the "global Exception" trick will only work for code in the current module. Exception is a builtin and if you really want to change it globally you need to import __builtin__; __builtin__.Exception = MyException

4. Even if you changed __builtin__.Exception it will only affect future uses of Exception, subclasses that have already been defined will use the original Exception class and will be unaffected by your changes. You could loop over Exception.__subclasses__ and change the __bases__ for each one of them to insert your Exception subclass there.

5. There are subclasses of Exception that are also built-in types that you also cannot modify, although I'm not sure you would want to hook any of them (think StopIterration).

I think that the only decent way to do what you want is to patch the Python sources.

Solution 3:

This code will not affect any exception classes that were created before the start of main, and most of the exceptions that happen will be of such kinds (KeyError, AttributeError, and so forth). And you can't really affect those "built-in exceptions" in the most important sense -- if anywhere in your code is e.g. a 1/0, the real ZeroDivisionError will be raised (by Python's own internals), not whatever else you may have bound to that exceptions' name.

So, I don't think your code can do what you want (despite all the semicolons, it's still supposed to be Python, right?) -- it could be done by patching the C sources for the Python runtime, essentially (e.g. by providing a hook potentially caught on any exception even if it's later caught) -- such a hook currently does not exist because the use cases for it would be pretty rare (for example, a StopIteration is always raised at the normal end of every for loop -- and caught, too; why on Earth would one want to trace that, and the many other routine uses of caught exceptions in the Python internals and standard library?!).