Exception Handling

Exception Policies

When an exception propagates outside of a hook handler, the hook group decides what to do according to its exception propagation strategy.

Setting the Active Policy

Setting the strategy is done with the gossip.set_exception_policy() function:

>>> import gossip
>>> gossip.set_exception_policy(gossip.RaiseDefer())
>>> gossip.set_exception_policy(gossip.RaiseImmediately())
>>> gossip.set_exception_policy(gossip.IgnoreExceptions())

The policy can be changed per each hook group. The default is to inherit the behavior of the parent group. gossip.set_exception_policy() is merely an alias for gossip.groups.Group.set_exception_policy().

Available Exception Handling Policies

The possible strategies that gossip.set_exception_policy() can receive are:

• gossip.RaiseImmediately(): Raise upon the first exception encountered. This is the default behavior for the global group, and causes handlers to be skipped upon error.
• gossip.RaiseDefer(): Call all handlers, but raise the first exception encountered. This swallows intermediate errors, but at least tries to call as many handlers as possible.
• gossip.IgnoreExceptions(): This does nothing upon encountering errors, and calls all handlers always.
• gossip.Inherit(): Takes the policy from the group’s parent. You cannot set this policy for the global group, as it has no parent. This is the default behavior for non-global groups.

Handling Exceptions via the Built-in Hooks

Gossip has predefined built-in hooks to handle various situations externally. One of those is gossip.on_handler_exception. This is called for every exception raised from a handler, and allows you to intervene before Gossip moves on. One useful way to use this is to fire a debugger:

>>> @gossip.register('gossip.on_handler_exception')
... def debugger(handler, exception, hook):
...    exc_type, exc_value, exc_tb = exception
...    import pdb
...    pdb.post_mortem(exc_tb)

Note

Any exception raised from a handler for gossip.on_handler_exception will be propagated immediately. This is considered an internal error and no recovery is attempted. Be very careful when writing these handlers.