.. _validating:

Debugging Validation Errors
===========================

.. note::

   This material should be expanded in a future release when
   :ticket:`trac/171 <171>` is addressed.

In the 1.1.x release series of PyXB a common and valid complaint was that
PyXB would validate documents and binding instances, but did not provide
useful information explaining why a particular document was invalid.  The
content validation approach described in :ref:`validating-content` allows
the application to diagnose:

- the element or instance data that failed to satisfy the content model;

- the location of that element within a document being parsed;

- the point in the XML schema where the containing complex type was defined;

- the content that was expected to have been encountered.

Perhaps the quickest way to see the information PyXB now provides is to look
at the :ref:`ex_ndfd` example, and in particular the ``latlon.py`` script.
This script uses WSDL to obtain the near-term forecast for a location within
the United States using latitude and longitude.

In the course of validating PyXB 1.2.0, it was discovered that the National
Weather Service has added some new elements to their schema, with the result
that the servers no longer provide valid XML: in particular, order
constraints on individual forecast characteristics are violated.  This
caused the example to fail.

While the script needs to be cleaned up considerably, the following code
block shows how the retrieved XML document is converted into a binding
instance:

.. code-block:: python

   # Create the binding instance from the response.  If there's a
   # problem, diagnose the issue, then try again with validation
   # disabled.
   r = None
   try:
       r = DWML.CreateFromDocument(rxml)
   except pyxb.UnrecognizedContentError as e:
       print '*** ERROR validating response:'
       print e.details()
   if r is None:
       pyxb.RequireValidWhenParsing(False)
       r = DWML.CreateFromDocument(rxml)
   
When invoking the program, the following diagnostic output is provided
through the :api:`details <pyxb.exceptions_.ValidationError.details>`
method:

.. code-block:: none
   :linenos:

   *** ERROR validating response:
   The containing element parameters is defined at ndfd_data.xsd[29:12].
   The containing element type parametersType is defined at parameters.xsd[21:4]
   The unrecognized content probability-of-precipitation begins at <unknown>[217:6]
   The parametersType automaton is in an accepting state.
   The following element and wildcard content would be accepted:
        An element cloud-amount per parameters.xsd[379:12]
        An element humidity per parameters.xsd[416:12]
        An element weather per parameters.xsd[449:12]
        An element conditions-icon per parameters.xsd[566:12]
        An element hazards per parameters.xsd[585:12]
        An element wordedForecast per parameters.xsd[701:11]
        An element pressure per parameters.xsd[715:12]
        An element probabilisticCondition per parameters.xsd[751:12]
        An element water-state per parameters.xsd[785:12]

This tells the user that:

2. A problem was encountered processing an element ``parameters`` from an
   element declaration found in the ``ndfd_data.xsd`` schema starting at
   column 12 on line 29.

3. The ``parameters`` element is of type ``parametersType``, and its content
   model can be found in the complex type definition in the
   ``parameters.xsd`` schema starting on line 21.

4. The error in the document was failure to recognize an element
   ``probability-of-precipitation`` which was found at line 217 of the
   input.  (The document is being parsed from a Python string, so no URI is
   available).

5. Had the containing ``parameters`` element ended before this point, the
   element would be valid.

6. The names and definition locations of element data that would have been
   acceptable at that point follow in lines 7 through 15.

A much shorter but still useful synopsis of the invalidity would be
available through the ``str`` operation on the exception.  Full details are
available through attributes on the :api:`UnrecognizedContentError
<pyxb.exceptions_.UnrecognizedContentError>` and other exceptions.

In cases where the service generating the documents is under your control,
you can use this information to correct the documents.  In cases like this
where the error is in a production server, the proper approach is to `report
the error
<http://tech.groups.yahoo.com/group/NDFD_SOAP_Service/message/2218>`_,
disable validation, and move on with ones life.

Runtime Exception Hierarchy
---------------------------

Details on the interfaces presented by these exceptions can be found through
the :api:`API Documentation <pyxb.exceptions_.ValidationError>`.

.. image:: Images/RuntimeExceptions.jpg

.. image:: Images/CTDValidationExceptions.jpg

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