.. _shape_info:
============================================
How Shape Information is Handled by PyTensor
============================================
Currently, information regarding shape is used in the following ways by PyTensor:
- 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
:meth:`Op.infer_shape` method.
- To generate faster compiled code (e.g. for a 2D convolution).
Example:
>>> import pytensor
>>> x = pytensor.tensor.matrix('x')
>>> f = pytensor.function([x], (x ** 2).shape)
>>> pytensor.dprint(f)
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 computations; PyTensor has removed them to compute the shape of the output
directly.
PyTensor propagates information about shapes within a graph using specialized
:class:`Op`\s and static :class:`Type` information (see :ref:`pytensor_type`).
Specifying Exact Shape
======================
You can create variables with static shape information as follows:
.. code-block:: python
pytensor.tensor.tensor("float64", shape=(4, 3, 2))
You can also pass shape information directly to some :class:`Op`\s, like ``RandomVariables``
.. code-block:: python
pytensor.tensor.random.normal(size=(7, 3, 5, 5))
- You can use the :class:`SpecifyShape`\ :class:`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 PyTensor function to a constant.
>>> import pytensor
>>> x = pytensor.tensor.matrix()
>>> x_specify_shape = pytensor.tensor.specify_shape(x, (2, 2))
>>> f = pytensor.function([x], (x_specify_shape ** 2).shape)
>>> pytensor.printing.debugprint(f) # doctest: +NORMALIZE_WHITESPACE
DeepCopyOp [id A] '' 0
|TensorConstant{(2,) of 2} [id B]
Problems with Shape inference
=============================
Sometimes this can lead to errors. Consider this example:
>>> import numpy as np
>>> import pytensor
>>> x = pytensor.tensor.matrix('x')
>>> y = pytensor.tensor.matrix('y')
>>> z = pytensor.tensor.join(0, x, y)
>>> xv = np.random.random((5, 4))
>>> yv = np.random.random((3, 3))
>>> f = pytensor.function([x, y], z.shape)
>>> pytensor.printing.debugprint(f) # doctest: +NORMALIZE_WHITESPACE
MakeVector{dtype='int64'} [id A] '' 4
|Elemwise{Add}[(0, 0)] [id B] '' 3
| |Shape_i{0} [id C] '' 2
| | |x [id D]
| |Shape_i{0} [id E] '' 1
| |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 = pytensor.function([x,y], z)# Do not take the shape.
>>> pytensor.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 above), 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 PyTensor variable, which leads to an error.
This might happen with other `Op`\s such as :class:`Elemwise` and :class:`Dot`, for example.
Indeed, to perform some optimizations/rewrites (for speed or stability, for instance),
PyTensor 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 PyTensor flag ``optimizer_excluding=local_shape_to_shape_i``. You can
also obtain the same effect by running in the modes ``FAST_COMPILE`` or
:class:`DebugMode`.