.. index:: debugging, exceptions, trace trait change events

=========================
Tips for debugging Traits
=========================


Re-raising exceptions in change handlers
========================================

Traits will typically log (instead of raise) exceptions when an exception is
encountered in a trait-change handler. This behavior is often preferred in
applications, since you usually want to avoid critical failures in
applications. However, when debugging these errors, the
``logging.Logger.exception`` only displays the tip of the traceback. For example,
the following code changes a ``constant``:

.. code-block:: python

   from traits.api import HasTraits, Int

   class Curmudgeon(HasTraits):
       constant = Int(1)
       def _constant_changed(self):
           raise ValueError()

   c = Curmudgeon()
   c.constant = 42

The ``constant`` trait-change handler raises an exception that is caught and
logged::

   Exception occurred in traits notification handler.
   Please check the log file for details.
   Exception occurred in traits notification handler for object:
   <__main__.Curmudgeon object at 0x107603050>, trait: constant, old value: 0, new value: 42
     ...
     File "curmudgeon.py", line 12, in _constant_changed
       raise ValueError()
   ValueError

This logged exception, however, only contains the tip of the traceback. This
makes debugging a bit difficult. You can force exceptions to be re-raised
by adding a custom exception handler:

.. code-block:: python

   from traits.api import push_exception_handler
   push_exception_handler(reraise_exceptions=True)

(For example, you could add this to the top of the original code block.)

Re-running the original code example with this custom handler will now raise
the following traceback::

   Traceback (most recent call last):
     File "curmudgeon.py", line 15, in <module>
       c.constant = 42
     ...
     File "curmudgeon.py", line 12, in _constant_changed
       raise ValueError()
   ValueError

Notice that this traceback has information about *where* we changed
``constant``.  Note: This is a toy example; use ``Constant`` from
``traits.api`` if you actually want a constant trait.


Tracing Traits Change Events
============================

Occasionally it is necessary to find the chain of event dispatches in traits
classes. To help with debugging, a |record_events| context manager is provided
in :mod:`traits.util.event_tracer`. Trait change events taking place inside the
context block will be recorded in a change event container (see example below)
and can be saved to files (a file for each thread) for further inspection.


Example:

.. code-block:: python

    from traits.api import *
    from traits.util.event_tracer import record_events


    class MyModel(HasTraits):

        number = Float(2.0)
        list_of_numbers = List(Float())
        count = Int(0)

        @on_trait_change('number')
        def _add_number_to_list(self, value):
            self.list_of_numbers.append(value)

        @on_trait_change('list_of_numbers[]')
        def _count_items(self):
            self.count = len(self.list_on_numbers)

        def add_to_number(self, value):
            self.number += value


    my_model = MyModel()

    with record_events() as change_event_container:
        my_model.number = 4.7
        my_model.number = 3

    # save files locally
    change_event_container.save_to_directory('./')


Running the above example will write a file named MAinThread.trace in the
local folder. The file contents will be similar to the lines below::

    2014-03-21 14:11:20.779000 -> 'number' changed from 2.0 to 4.7 in 'MyModel'
    2014-03-21 14:11:20.779000     CALLING: '_add_number_to_list' in example.py
    2014-03-21 14:11:20.780000 ---> 'list_of_numbers_items' changed from <undefined> to <traits.trait_handlers.TraitListEvent object at 0x03C85AF0> in 'MyModel'
    2014-03-21 14:11:20.780000       CALLING: 'handle_list_items_special' in C:\Users\itziakos\Projects\traits\traits\traits_listener.py
    2014-03-21 14:11:20.780000 -----> 'list_of_numbers_items' changed from [] to [4.7] in 'MyModel'
    2014-03-21 14:11:20.780000         CALLING: '_count_items' in exampler.py
    2014-03-21 14:11:20.780000 -------> 'trait_added' changed from <undefined> to 'list_on_numbers' in 'MyModel'
    2014-03-21 14:11:20.780000           CALLING: '_trait_added_changed' in C:\Users\itziakos\Projects\traits\traits\has_traits.py
    2014-03-21 14:11:20.780000 <------- EXIT: '_trait_added_changed'
    2014-03-21 14:11:20.780000 <----- EXIT: '_count_items' [EXCEPTION: 'MyModel' object has no attribute 'list_on_numbers']
    2014-03-21 14:11:20.780000 <--- EXIT: 'handle_list_items_special'
    2014-03-21 14:11:20.781000 <- EXIT: '_add_number_to_list'

    2014-03-21 14:11:20.781000 -> 'number' changed from 4.7 to 3.0 in 'MyModel'
    2014-03-21 14:11:20.781000     CALLING: '_add_number_to_list' in example.py
    2014-03-21 14:11:20.781000 ---> 'list_of_numbers_items' changed from <undefined> to <traits.trait_handlers.TraitListEvent object at 0x03C85A30> in 'MyModel'
    2014-03-21 14:11:20.781000       CALLING: 'handle_list_items_special' in C:\Users\itziakos\Projects\traits\traits\traits_listener.py
    2014-03-21 14:11:20.781000 -----> 'list_of_numbers_items' changed from [] to [3.0] in 'MyModel'
    2014-03-21 14:11:20.781000         CALLING: '_count_items' in example.py
    2014-03-21 14:11:20.781000 <----- EXIT: '_count_items' [EXCEPTION: 'MyModel' object has no attribute 'list_on_numbers']
    2014-03-21 14:11:20.782000 <--- EXIT: 'handle_list_items_special'
    2014-03-21 14:11:20.782000 <- EXIT: '_add_number_to_list'


.. |record_events| replace:: :func:`~traits.util.event_tracer.record_events`
