.. _shape_info:

==========================================
How Shape Information is Handled by Theano
==========================================

It is not possible to strictly enforce the shape of a Theano variable when
building a graph since the particular value provided at run-time for a parameter of a
Theano function may condition the shape of the Theano variables in its graph.

Currently, information regarding shape is used in two ways in Theano:

- To generate faster C code for the 2d convolution on the CPU and the GPU,
  when the exact output shape is known in advance.

- To remove computations in the graph when we only want to know the
  shape, but not the actual value of a variable. This is done with the
  `Op.infer_shape <http://deeplearning.net/software/theano/extending/cop.html#Op.infer_shape>`_
  method.

  Example:

>>> import theano
>>> x = theano.tensor.matrix('x')
>>> f = theano.function([x], (x ** 2).shape)
>>> theano.printing.debugprint(f) # doctest: +NORMALIZE_WHITESPACE
MakeVector{dtype='int64'} [id A] ''   2
 |Shape_i{0} [id B] ''   1
 | |x [id C]
 |Shape_i{1} [id D] ''   0
   |x [id C]
    

The output of this compiled function does not contain any multiplication
or power. Theano has removed them to compute directly the shape of the
output.

Shape Inference Problem
=======================

Theano propagates information about shape in the graph. Sometimes this
can lead to errors. Consider this example:

>>> import numpy
>>> import theano
>>> x = theano.tensor.matrix('x')
>>> y = theano.tensor.matrix('y')
>>> z = theano.tensor.join(0, x, y)
>>> xv = numpy.random.rand(5, 4)
>>> yv = numpy.random.rand(3, 3)

>>> f = theano.function([x, y], z.shape)
>>> theano.printing.debugprint(f) # doctest: +NORMALIZE_WHITESPACE
MakeVector{dtype='int64'} [id A] ''   4
 |Elemwise{Add}[(0, 0)] [id B] ''   3
 | |Shape_i{0} [id C] ''   1
 | | |x [id D]
 | |Shape_i{0} [id E] ''   2
 |   |y [id F]
 |Shape_i{1} [id G] ''   0
   |x [id D]

>>> f(xv, yv) # DOES NOT RAISE AN ERROR AS SHOULD BE.
array([8, 4])

>>> f = theano.function([x,y], z)# Do not take the shape.
>>> theano.printing.debugprint(f) # doctest: +NORMALIZE_WHITESPACE
Join [id A] ''   0
 |TensorConstant{0} [id B]
 |x [id C]
 |y [id D]

>>> f(xv, yv)  # doctest: +ELLIPSIS
Traceback (most recent call last):
  ...
ValueError: ...

As you can see, when asking only for the shape of some computation (``join`` in the
example), an inferred shape is computed directly, without executing
the computation itself (there is no ``join`` in the first output or debugprint).

This makes the computation of the shape faster, but it can also hide errors. In
this example, the computation of the shape of the output of ``join`` is done only
based on the first input Theano variable, which leads to an error.

This might happen with other ops such as ``elemwise`` and ``dot``, for example.
Indeed, to perform some optimizations (for speed or stability, for instance),
Theano assumes that the computation is correct and consistent
in the first place, as it does here.

You can detect those problems by running the code without this
optimization, using the Theano flag
``optimizer_excluding=local_shape_to_shape_i``. You can also obtain the
same effect by running in the modes ``FAST_COMPILE`` (it will not apply this
optimization, nor most other optimizations) or ``DebugMode`` (it will test
before and after all optimizations (much slower)).


Specifing Exact Shape
=====================

Currently, specifying a shape is not as easy and flexible as we wish and we plan some
upgrade.  Here is the current state of what can be done:

- You can pass the shape info directly to the ``ConvOp`` created
  when calling ``conv2d``. You simply set the parameters ``image_shape``
  and ``filter_shape`` inside the call. They must be tuples of 4
  elements. For example:

.. code-block:: python

    theano.tensor.nnet.conv2d(..., image_shape=(7, 3, 5, 5), filter_shape=(2, 3, 4, 4))

- You can use the ``SpecifyShape`` op to add shape information anywhere in the
  graph. This allows to perform some optimizations. In the following example,
  this makes it possible to precompute the Theano function to a constant.


>>> import theano
>>> x = theano.tensor.matrix()
>>> x_specify_shape = theano.tensor.specify_shape(x, (2, 2))
>>> f = theano.function([x], (x_specify_shape ** 2).shape)
>>> theano.printing.debugprint(f) # doctest: +NORMALIZE_WHITESPACE
DeepCopyOp [id A] ''   0
 |TensorConstant{(2,) of 2} [id B]

Future Plans
============

  The parameter "constant shape" will be added to ``theano.shared()``. This is probably
  the most frequent occurrence with ``shared`` variables. It will make the code
  simpler and will make it possible to check that the shape does not change when
  updating the ``shared`` variable.
