
.. _debugmode:

=================
:mod:`debugmode`
=================

.. module:: debugmode
   :platform: Unix, Windows
   :synopsis: defines DebugMode
.. moduleauthor:: LISA

Guide
=====


The DebugMode evaluation mode includes a number of self-checks and assertions
that can help to diagnose several kinds of programmer errors that can lead to
incorrect output.

It is much slower to evaluate a function or method with DebugMode than
it would be in ``'FAST_RUN'`` or even ``'FAST_COMPILE'``.  We recommended you use
DebugMode during development, but not when you launch 1000 processes on
a cluster.

DebugMode can be used as follows:

.. code-block:: python

    x = tensor.dscalar('x')

    f = theano.function([x], 10*x, mode='DebugMode')

    f(5)
    f(0)
    f(7)

It can also be used by setting the configuration variable :attr:`config.mode`.
It can also be used by passing a DebugMode instance as the mode, as in 

>>> f = theano.function([x], 10*x, mode=DebugMode(check_c_code=False))

If any problem is detected, DebugMode will raise an exception according to
what went wrong, either at call time (``f(5)``) or compile time (
``f = theano.function(x, 10*x, mode='DebugMode')``). These exceptions
should *not* be ignored; talk to your local Theano guru or email the
users list if you cannot make the exception go away.

Some kinds of errors can only be detected for certain input value combinations.
In the example above, there is no way to guarantee that a future call to say,
``f(-1)`` won't cause a problem.  DebugMode is not a silver bullet.

If you instantiate DebugMode using the constructor ``compile.DebugMode``
rather than the keyword ``DebugMode`` you can configure its behaviour via
constructor arguments. 

Reference
==========

.. class:: DebugMode(Mode)

    Evaluation Mode that detects internal theano errors.

    This mode catches several kinds of internal error:

    - inconsistent outputs when calling the same Op twice with the same
      inputs, for instance if c_code and perform implementations, are
      inconsistent, or in case of incorrect handling of output memory
      (see `BadThunkOutput`)

    - a variable replacing another when their runtime values don't match.  This is a symptom of
      an incorrect optimization step, or faulty Op implementation (raises `BadOptimization`)

    - stochastic optimization ordering (raises `StochasticOrder`)

    - incomplete `destroy_map` specification (raises `BadDestroyMap`)

    - an op that returns an illegal value not matching the output Variable Type (raises
      InvalidValueError)

    Each of these exceptions inherits from the more generic `DebugModeError`.

    If there are no internal errors, this mode behaves like FAST_RUN or FAST_COMPILE, but takes
    a little longer and uses more memory.

    If there are internal errors, this mode will raise an `DebugModeError` exception.

    .. attribute:: stability_patience = config.DebugMode.patience

        When checking for the stability of optimization, recompile the graph this many times.
        Default 10.

    .. attribute:: check_c_code = config.DebugMode.check_c

        Should we evaluate (and check) the `c_code` implementations?

        ``True`` -> yes, ``False`` -> no.

        Default yes.

    .. attribute:: check_py_code = config.DebugMode.check_py

    Should we evaluate (and check) the `perform` implementations?

        ``True`` -> yes, ``False`` -> no.

        Default yes.

    .. attribute:: check_isfinite = config.DebugMode.check_finite

        Should we check for (and complain about) ``NaN``/``Inf`` ndarray elements?

        ``True`` -> yes, ``False`` -> no.

        Default yes.

    .. attribute:: require_matching_strides = config.DebugMode.check_strides

        Check for (and complain about) Ops whose python and C
        outputs are ndarrays with different strides. (This can catch bugs, but
        is generally overly strict.)

        0 -> no check, 1 -> warn, 2 -> err.

        Default warn.

    .. method:: __init__(self, optimizer='fast_run', stability_patience=None, check_c_code=None, check_py_code=None, check_isfinite=None, require_matching_strides=None, linker=None)

        Initialize member variables.

        If any of these arguments (except optimizer) is not None, it overrides the class default.
        The linker arguments is not used. It is set their to allow Mode.requiring() and some other fct to work with DebugMode too.



The keyword version of DebugMode (which you get by using ``mode='DebugMode``)
is quite strict, and can raise several different Exception types.
There following are DebugMode exceptions you might encounter:


.. class:: DebugModeError(Exception)

    This is a generic error.  All the other exceptions inherit from this one.
    This error is typically not raised directly.
    However, you can use ``except DebugModeError: ...`` to catch any of the more
    specific types of Exception.



.. class:: BadThunkOutput(DebugModeError)

    This exception means that different calls to the same Op with the same
    inputs did not compute the same thing like they were supposed to.
    For instance, it can happen if the python (``perform``) and c (``c_code``)
    implementations of the Op are inconsistent (the problem might be a bug in
    either ``perform`` or ``c_code`` (or both)).  It can also happen if
    ``perform`` or ``c_code`` does not handle correctly output memory that
    has been preallocated (for instance, if it did not clear the memory before
    accumulating into it, or if it assumed the memory layout was C-contiguous
    even if it is not).



.. class:: BadOptimization(DebugModeError)

    This exception indicates that an Optimization replaced one variable (say V1)
    with another one (say V2)  but at runtime, the values for V1 and V2 were
    different.  This is something that optimizations are not supposed to do.

    It can be tricky to identify the one-true-cause of an optimization error, but
    this exception provides a lot of guidance.  Most of the time, the
    exception object will indicate which optimization was at fault.
    The exception object also contains information such as a snapshot of the
    before/after graph where the optimization introduced the error.



.. class:: BadDestroyMap(DebugModeError)

    This happens when an Op's ``perform()`` or ``c_code()`` modifies an input that it wasn't
    supposed to.  If either the ``perform`` or ``c_code`` implementation of an Op
    might modify any input, it has to advertise that fact via the ``destroy_map``
    attribute.

    For detailed documentation on the ``destroy_map`` attribute, see :ref:`inplace`.


.. class:: BadViewMap(DebugModeError)

    This happens when an Op's perform() or c_code() creates an alias or alias-like
    dependency between an input and an output... and it didn't warn the
    optimization system via the ``view_map`` attribute.

    For detailed documentation on the ``view_map`` attribute, see :ref:`views`.


.. class:: StochasticOrder(DebugModeError)

    This happens when an optimization does not perform the same graph operations
    in the same order when run several times in a row.  This can happen if any
    steps are ordered by ``id(object)`` somehow, such as via the default object
    hash function.  A Stochastic optimization invalidates the pattern of work
    whereby we debug in DebugMode and then run the full-size jobs in FAST_RUN.


.. class:: InvalidValueError(DebugModeError)

    This happens when some Op's ``perform`` or ``c_code`` implementation computes
    an output that is invalid with respect to the type of the corresponding output
    variable.  Like if it returned a complex-valued ndarray for a ``dscalar``
    Type.

    This can also be triggered when floating-point values such as NaN and Inf are
    introduced into the computations.  It indicates which Op created the first
    NaN.  These floating-point values can be allowed by passing the
    ``check_isfinite=False`` argument to DebugMode. 



