
.. _tutorial_graphstructures:

================
Graph Structures
================

Debugging or profiling code written in Theano is not that simple if you
do not know what goes on under the hood. This chapter is meant to
introduce you to a required minimum of the inner workings of Theano, 
for more details see :ref:`extending`.

The first step in writing Theano code is to write down all mathematical 
relations using symbolic placeholders (**variables**). When writing down 
these expressions you use operations like ``+``, ``-``, ``**``,
``sum()``, ``tanh()``. All these are represented internally as **ops**. 
An **op** represents a certain computation on some type of inputs
producing some type of output. You can see it as a function definition
in most programming languages. 

Theano builds internally a graph structure composed of interconnected 
**variable** nodes, **op** nodes and **apply** nodes. An 
**apply** node represents the application of an **op** to some 
**variables**. It is important to make the difference between the
definition of a computation represented by an **op** and its application
to some actual data which is represented by the **apply** node. For more
details about these building blocks see :ref:`variable`, :ref:`op`, 
:ref:`apply`. A graph example is the following:


**Code**

.. code-block:: python

    x = T.dmatrix('x')
    y = T.dmatrix('y')
    z = x + y

**Diagram**

.. _tutorial-graphfigure: 

.. figure:: apply.png 
    :align: center

    Interaction between instances of Apply (blue), Variable (red), Op (green),
    and Type (purple).

.. # COMMENT
    WARNING: hyper-links and ref's seem to break the PDF build when placed
    into this figure caption.

Arrows in this :ref:`figure <tutorial-graphfigure>` represent references to the 
Python objects pointed at. The blue
box is an :ref:`apply` node. Red boxes are :ref:`variable` nodes. Green
circles are :ref:`Ops <op>`. Purple boxes are :ref:`Types <type>`.


The graph can be traversed starting from outputs (the result of some
computation) down to its inputs using the owner field.
Take for example the following code:

.. code-block:: python

    x = T.dmatrix('x')
    y = x*2.

If you print `type(y.owner)`` you get ``<class 'theano.gof.graph.Apply'>``, 
which is the apply node that connects the op and the inputs to get this
output. You can now print the name of the op that is applied to get 
``y``:

>>> y.owner.op.name
'Elemwise{mul,no_inplace}'

So a elementwise multiplication is used to compute ``y``. This
muliplication is done between the inputs

>>> len(y.owner.inputs)
2
>>> y.owner.inputs[0]
x
>>> y.owner.inputs[1]
InplaceDimShuffle{x,x}.0

Note that the second input is not 2 as we would have expected. This is 
because 2 was first :term:`broadcasted <broadcasting>` to a matrix of 
same shape as x. This is done by using the op ``DimShuffle`` :

>>> type(y.owner.inputs[1])
<class 'theano.tensor.basic.TensorVariable'>
>>> type(y.owner.inputs[1].owner)
<class 'theano.gof.graph.Apply'>
>>> y.owner.inputs[1].owner.op
<class 'theano.tensor.elemwise.DimShuffle object at 0x14675f0'>
>>> y.owner.inputs[1].owner.inputs
[2.0]


Starting from this graph structure is easy to understand how 
*automatic differentiation* is done, or how the symbolic relations
can be optimized for performance or stability.


Automatic Differentiation
=========================

Having the graph structure, computing automatic differentiation is
simple. The only thing :func:`tensor.grad` has to do is to traverse the
graph from the outputs back towards the inputs through all :ref:`apply`
nodes ( :ref:`apply` nodes are those who define what computations the
graph does). For each such :ref:`apply` node, its  :ref:`op` defines 
how to compute the gradient of the node's outputs with respect to its
inputs. Note that if an :ref:`op` does not provide this information, 
it is assumed that the gradient does not defined.
Using the 
`chain rule <http://en.wikipedia.org/wiki/Chain_rule>`_ 
these gradients can be composed in order to obtain the expression of the 
gradient of the graph's output with respect to the graph's inputs .


Optimizations
=============

When compiling a Theano function, what you give to the
:func:`theano.function <function.function>` is actually a graph
(starting from the outputs variables you can traverse the graph up to
the input variables). While this graph structure shows how to compute
the output from the input, it also offers the posibility to improve the  
the way this computation is carried out. The way optimizations work in 
Theano is by indentifying and replacing certain patterns in the graph 
with other specialized patterns that produce the same results but are either 
faster or more stable. Optimizations can also detect 
identical subgraphs and ensure that the same values are not computed
twice or reformulate parts of the graph to a GPU specific version.

For example, one (simple) optimization that Theano uses is to replace 
the pattern :math:`\frac{xy}{y}` by :math:`x`.
