change_event_hooks_example.py

from contextlib import contextmanager
import inspect

from traits.api import *
from traits import trait_notifiers


class ChangeEventPrinter(object):

    def __init__(self):
        self.indent = 1

    def pre_tracer(self, obj, name, old, new, handler):

        change_pfx = '-' * self.indent*2 + '>'
        change_msg = "{pfx} '{name}' changed from {old} to {new} in '{class_name}'"
        print change_msg.format(pfx=change_pfx, name=name,
                                old=old, new=new,
                                class_name=obj.__class__.__name__)

        calling_pfx = ' '*(self.indent*2+1)
        calling_msg = "{pfx} CALLING: '{handler}' in {source}"
        print calling_msg.format(pfx=calling_pfx, handler=handler.__name__,
                                 source=inspect.getsourcefile(handler))

        self.indent += 1

    def post_tracer(self, obj, name, old, new, handler, exception=None):
        self.indent -= 1

        pfx = '<' + '-' * self.indent*2

        if exception:
            exception_msg = ' [EXCEPTION: {}]'.format(exception)
        else:
            exception_msg = ''

        exit_msg = "{pfx} EXIT: '{handler}'{exception}"
        print exit_msg.format(pfx=pfx, handler=handler.__name__,
                              exception=exception_msg)

        if self.indent == 1:
            print

@contextmanager
def print_change_events():
    printer = ChangeEventPrinter()
    trait_notifiers.set_change_event_tracers(pre_tracer=printer.pre_tracer,
                                             post_tracer=printer.post_tracer)

    try:
        yield
    finally:
        trait_notifiers.clear_change_event_tracers()


class Foo(HasTraits):
    """ Test traits class with static and dynamic listeners.

    Changing `baz` triggers a dynamic listeners that modifies `bar`, which
    triggers one dynamic and one static listeners.
    """

    bar = Float
    baz = Float
    fuz = Float

    def _bar_changed(self):
        pass

    @on_trait_change('bar')
    def _on_bar_change_notification(self):
        pass

    @on_trait_change('baz')
    def _on_baz_change_notification(self):
        self.bar += 1

    @on_trait_change('fuz')
    def _on_fuz_change_notification(self):
        self.bar += 1
        raise Exception('Crash')


if __name__ == '__main__':
    foo = Foo(bar=3)
    with print_change_events():
        foo.baz = 4
        foo.fuz = 7