Debugging
=========

While it is often the case that a network works perfectly on the first
try, bugs do happen. 

Gradient checking
-----------------

If you add your own expression operations, you can debug the automatic
differentiation using::

    check_gradients(e)

Printing expressions
--------------------

If you print out an Expression object, you'll see the top-level
operation in the expression. You won't see its value, or even its
shape/dtype::

    >>> print constant(1)+constant(1)
    add(<0>, <0>)

However, there's (usually) no harm in calling ``compute_value`` in the
middle of your code::

    >>> print compute_value(constant(1)+constant(1))
    2

This will compute the value of x and all its subexpressions, and
return the value of x.


Tracing
-------

::

    debug.trace = True

This lets you see the value and gradient of every subexpression::

    >>> compute_value(constant(1)+constant(1))
    <0> = constant(1) = array(1)
    <1> = constant(1) = array(1)
    <2> = add(<0>, <1>) = 2
    2

Expression tracebacks
---------------------

::

    debug.stack = True

If an exception is raised while computing a value or gradient, the
usual traceback is not very helpful. This option turns on an
"expression traceback" that shows you the line that created the
offending expression.