# Graph Structures¶

Aesara works by modeling mathematical operations and their outputs using symbolic placeholders, or variables, which inherit from the class Variable. When writing expressions in Aesara one uses operations like +, -, **, sum(), tanh(). These are represented internally as Ops. An Op represents a computation that is performed on a set of symbolic inputs and produces a set of symbolic outputs. These symbolic input and output Variables carry information about their types, like their data type (e.g. float, int), the number of dimensions, etc.

Aesara graphs are composed of interconnected Apply, Variable and Op nodes. An Apply node represents the application of an Op to specific Variables. It is important to draw the difference between the definition of a computation represented by an Op and its application to specific inputs, which is represented by the Apply node.

The following illustrates these elements:

Code

import aesara.tensor as at

x = at.dmatrix('x')
y = at.dmatrix('y')
z = x + y


Diagram The blue box is an Apply node. Red boxes are Variables. Green circles are Ops. Purple boxes are Types.

When we create Variables and then Apply Ops to them to make more Variables, we build a bi-partite, directed, acyclic graph. Variables point to the Apply nodes representing the function application producing them via their Variable.owner field. These Apply nodes point in turn to their input and output Variables via their Apply.inputs and Apply.outputs fields.

The Variable.owner field of both x and y point to None because they are not the result of another computation. If one of them was the result of another computation, its Variable.owner field would point to another blue box like z does, and so on.

## Traversing the graph¶

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:

>>> import aesara
>>> x = aesara.tensor.dmatrix('x')
>>> y = x * 2.


If you enter type(y.owner) you get <class 'aesara.graph.basic.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}'


Hence, an element-wise multiplication is used to compute y. This multiplication is done between the inputs:

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


Note that the second input is not 2 as we would have expected. This is because 2 was first broadcasted to a matrix of same shape as x. This is done by using the OpDimShuffle:

>>> type(y.owner.inputs)
<class 'aesara.tensor.var.TensorVariable'>
>>> type(y.owner.inputs.owner)
<class 'aesara.graph.basic.Apply'>
>>> y.owner.inputs.owner.op
<aesara.tensor.elemwise.DimShuffle object at 0x106fcaf10>
>>> y.owner.inputs.owner.inputs
[TensorConstant{2.0}]


All of the above can be succinctly summarized with the aesara.dprint() function:

>>> aesara.dprint(y)
Elemwise{mul,no_inplace} [id A] ''
|x [id B]
|InplaceDimShuffle{x,x} [id C] ''
|TensorConstant{2.0} [id D]


Starting from this graph structure it is easier to understand how automatic differentiation proceeds and how the symbolic relations can be rewritten for performance or stability.

## Graph Structures¶

The following section outlines each type of structure that may be used in an Aesara-built computation graph.

### Apply¶

An Apply node is a type of internal node used to represent a computation graph in Aesara. Unlike Variable, Apply nodes are usually not manipulated directly by the end user. They may be accessed via the Variable.owner field.

An Apply node is typically an instance of the Apply class. It represents the application of an Op on one or more inputs, where each input is a Variable. By convention, each Op is responsible for knowing how to build an Apply node from a list of inputs. Therefore, an Apply node may be obtained from an Op and a list of inputs by calling Op.make_node(*inputs).

Comparing with the Python language, an Apply node is Aesara’s version of a function call whereas an Op is Aesara’s version of a function definition.

An Apply instance has three important fields:

op
An Op that determines the function/transformation being applied here.
inputs
A list of Variables that represent the arguments of the function.
outputs
A list of Variables that represent the return values of the function.

An Apply instance can be created by calling graph.basic.Apply(op, inputs, outputs).

### Op¶

An Op in Aesara defines a certain computation on some types of inputs, producing some types of outputs. It is equivalent to a function definition in most programming languages. From a list of input Variables and an Op, you can build an Apply node representing the application of the Op to the inputs.

It is important to understand the distinction between an Op (the definition of a function) and an Apply node (the application of a function). If you were to interpret the Python language using Aesara’s structures, code going like def f(x): ... would produce an Op for f whereas code like a = f(x) or g(f(4), 5) would produce an Apply node involving the f Op.

### Type¶

A Type in Aesara provides static information (or constraints) about data objects in a graph. The information provided by Types allows Aesara to perform optimizations and produce more efficient compiled code.

Every symbolic Variable in an Aesara graph has an associated Type instance, and Types also serve as a means of constructing Variable instances. In other words, Types and Variables go hand-in-hand.

For example, aesara.tensor.irow is an instance of a Type and it can be used to construct variables as follows:

>>> from aesara.tensor import irow
>>> irow()
<TensorType(int32, (1, None))>


As the string print-out shows, irow specifies the following information about the Variables it constructs:

1. They represent tensors that are backed by numpy.ndarrays. This comes from the fact that irow is an instance of TensorType, which is the base Type for symbolic numpy.ndarrays.
2. They represent arrays of 32-bit integers (i.e. from the int32).
3. They represent arrays with shapes of , or, in code, (1, None), where None represents any shape value.

Note that Aesara Types are not necessarily equivalent to Python types or classes. Aesara’s TensorType’s, like irow, use numpy.ndarray as the underlying Python type for performing computations and storing data, but numpy.ndarrays model a much wider class of arrays than most TensorTypes. In other words, Aesara Type’s try to be more specific.

### Variable¶

A Variable is the main data structure you work with when using Aesara. The symbolic inputs that you operate on are Variables and what you get from applying various Ops to these inputs are also Variables. For example, when one inputs

>>> import aesara
>>> x = aesara.tensor.ivector()
>>> y = -x


x and y are both Variables. The Type of both x and y is aesara.tensor.ivector.

Unlike x, y is a Variable produced by a computation (in this case, it is the negation of x). y is the Variable corresponding to the output of the computation, while x is the Variable corresponding to its input. The computation itself is represented by another type of node, an Apply node, and may be accessed through y.owner.

More specifically, a Variable is a basic structure in Aesara that represents a datum at a certain point in computation. It is typically an instance of the class Variable or one of its subclasses.

A Variable r contains four important fields:

type
a Type defining the kind of value this Variable can hold in computation.
owner
this is either None or an Apply node of which the Variable is an output.
index
the integer such that owner.outputs[index] is r (ignored if Variable.owner is None)
name
a string to use in pretty-printing and debugging.

Variable has an important subclass: Constant.

#### Constant¶

A Constant is a Variable with one extra, immutable field: Constant.data. When used in a computation graph as the input of an OpApply, it is assumed that said input will always take the value contained in the Constant’s data field. Furthermore, it is assumed that the Op will not under any circumstances modify the input. This means that a Constant is eligible to participate in numerous optimizations: constant in-lining in C code, constant folding, etc.

## Automatic Differentiation¶

Having the graph structure, computing automatic differentiation is simple. The only thing aesara.grad() has to do is to traverse the graph from the outputs back towards the inputs through all Apply nodes. For each such Apply node, its Op defines how to compute the gradient of the node’s outputs with respect to its inputs. Note that if an Op does not provide this information, it is assumed that the gradient is not defined.

Using the 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.

A following section of this tutorial will examine the topic of differentiation in greater detail.

## Optimizations¶

When compiling an Aesara graph using aesara.function(), a graph is necessarily provided. While this graph structure shows how to compute the output from the input, it also offers the possibility to improve the way this computation is carried out. The way optimizations work in Aesara is by identifying 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.

For example, one (simple) optimization that Aesara uses is to replace the pattern by .

Example

Consider the following example of optimization:

>>> import aesara
>>> a = aesara.tensor.vector("a")      # declare symbolic variable
>>> b = a + a ** 10                    # build symbolic expression
>>> f = aesara.function([a], b)        # compile function
>>> print(f([0, 1, 2]))                # prints array([0,2,1026])
[    0.     2.  1026.]
>>> aesara.printing.pydotprint(b, outfile="./pics/symbolic_graph_unopt.png", var_with_name_simple=True)
The output file is available at ./pics/symbolic_graph_unopt.png
>>> aesara.printing.pydotprint(f, outfile="./pics/symbolic_graph_opt.png", var_with_name_simple=True)
The output file is available at ./pics/symbolic_graph_opt.png


We used aesara.printing.pydotprint() to visualize the optimized graph (right), which is much more compact than the unoptimized graph (left).

Unoptimized graph   Optimized graph  