# Registering callbacks

Extension code can register callbacks functions which will be called on specific events within the IPython code. You can see the current list of available callbacks, and the parameters that will be passed with each, in the callback prototype functions defined in `IPython.core.callbacks`.

To register callbacks, use `IPython.core.events.EventManager.register()`. For example:

    class VarWatcher(object):
        def __init__(self, ip):
            self.shell = ip
            self.last_x = None

        def pre_execute(self):
            self.last_x = self.shell.user_ns.get('x', None)

        def post_execute(self):
            if self.shell.user_ns.get('x', None) != self.last_x:
                print("x changed!")

    def load_ipython_extension(ip):
        vw = VarWatcher(ip)
        ip.events.register('pre_execute', vw.pre_execute)
        ip.events.register('post_execute', vw.post_execute)

Note

This API is experimental in IPython 2.0, and may be revised in future versions.

See also

Module `IPython.core.hooks`
  The older ‘hooks’ system allows end users to customise some parts of IPython’s behaviour.
Custom input transformation
  By registering input transformers that don’t change code, you can monitor what is being executed.