Adding a function to sys.excepthook

Question

Say I have something like this that sends unhanded exceptions to logging.critical() :

 import sys def register_handler(): orig_excepthook = sys.excepthook def error_catcher(*exc_info): import logging log = logging.getLogger(__name__) log.critical("Unhandled exception", exc_info=exc_info) orig_excepthook(*exc_info) sys.excepthook = error_catcher 

Works:

 import logging logging.basicConfig() register_handler() undefined() # logs, then runs original excepthook 

However, if register_handler() is called multiple times, several error_catcher is called in the chain, and the registration message appears several times.

I can come up with several methods, but none of them are particularly good (for example, if the sys.excepthook function is an error_catcher function or uses the has_registered attribute on a module to avoid double registration)

Is there a recommended way to do this?

Answer 1

The presence at the module level of an “already registered hook” variable seems to be the easiest and most reliable way to do this.

Other possible solutions will fall in certain (rather obscure) circumstances - if the sys.excepthook function is a built-in function, if the application registers a custom excepthook while maintaining the original excepthook during function definition, clobber subsequently registers the excepthook functions.

 import sys _hook_registered = False def register_handler(force = False): global _hook_registered if _hook_registered and not force: return orig_excepthook = sys.excepthook def error_catcher(*exc_info): import logging log = logging.getLogger(__name__) log.critical("Unhandled exception", exc_info=exc_info) orig_excepthook(*exc_info) sys.excepthook = error_catcher _hook_registered = True 

Answer 2

You can simply check if there is still a sys.excepthook built-in function before registering the handler:

 >>> import sys, types >>> isinstance(sys.excepthook, types.BuiltinFunctionType) True >>> sys.excepthook = lambda x: x >>> isinstance(sys.excepthook, types.BuiltinFunctionType) False 

Answer 3

If you put the code in your question into a module, you can import it many times, but it will only be executed for the first time.

Answer 4

If you make an orig_excepthook argument with a default value, the default value is fixed once during the definition. Therefore, repeated calls to register_handler will not change orig_excepthook .

 import sys def register_handler(orig_excepthook=sys.excepthook): def error_catcher(*exc_info): import logging log = logging.getLogger(__name__) log.critical("Unhandled exception", exc_info=exc_info) orig_excepthook(*exc_info) sys.excepthook = error_catcher 

---

 import logging logging.basicConfig() register_handler() register_handler() register_handler() undefined() 

issues only one call to log.critical .