Managing Expressions

Creating a String Representation of an Expression

There are several ways that string representations can be created from an expression, but the expression_to_string function provides the most flexible mechanism for generating a string representation. The options to this function control distinct aspects of the string representation.

Algebraic vs. Nested Functional Form

The default string representation is an algebraic form, which closely mimics the Python operations used to construct an expression. The verbose flag can be set to True to generate a string representation that is a nested functional form. For example:

import pyomo.core.expr as EXPR

M = ConcreteModel()
M.x = Var()

e = sin(M.x) + 2 * M.x

# sin(x) + 2*x
print(EXPR.expression_to_string(e))

# sum(sin(x), prod(2, x))
print(EXPR.expression_to_string(e, verbose=True))

Labeler and Symbol Map

The string representation used for variables in expression can be customized to define different label formats. If the labeler option is specified, then this function (or class functor) is used to generate a string label used to represent the variable. Pyomo defines a variety of labelers in the pyomo.core.base.label module. For example, the NumericLabeler defines a functor that can be used to sequentially generate simple labels with a prefix followed by the variable count:

import pyomo.core.expr as EXPR

M = ConcreteModel()
M.x = Var()
M.y = Var()

e = sin(M.x) + 2 * M.y

# sin(x1) + 2*x2
print(EXPR.expression_to_string(e, labeler=NumericLabeler('x')))

The smap option is used to specify a symbol map object (SymbolMap), which caches the variable label data. This option is normally specified in contexts where the string representations for many expressions are being generated. In that context, a symbol map ensures that variables in different expressions have a consistent label in their associated string representations.

Other Ways to Generate String Representations

There are two other standard ways to generate string representations:

  • Call the __str__() magic method (e.g. using the Python str() function. This calls expression_to_string, using the default values for all arguments.

  • Call the to_string() method on the ExpressionBase class. This calls expression_to_string and accepts the same arguments.

Evaluating Expressions

Expressions can be evaluated when all variables and parameters in the expression have a value. The value function can be used to walk the expression tree and compute the value of an expression. For example:

M = ConcreteModel()
M.x = Var()
M.x.value = math.pi / 2.0
val = value(M.x)
assert isclose(val, math.pi / 2.0)

Additionally, expressions define the __call__() method, so the following is another way to compute the value of an expression:

val = M.x()
assert isclose(val, math.pi / 2.0)

If a parameter or variable is undefined, then the value function and __call__() method will raise an exception. This exception can be suppressed using the exception option. For example:

M = ConcreteModel()
M.x = Var()
val = value(M.x, exception=False)
assert val is None

This option is useful in contexts where adding a try block is inconvenient in your modeling script.

Note

Both the value function and __call__() method call the evaluate_expression function. In practice, this function will be slightly faster, but the difference is only meaningful when expressions are evaluated many times.

Identifying Components and Variables

Expression transformations sometimes need to find all nodes in an expression tree that are of a given type. Pyomo contains two utility functions that support this functionality. First, the identify_components function is a generator function that walks the expression tree and yields all nodes whose type is in a specified set of node types. For example:

import pyomo.core.expr as EXPR

M = ConcreteModel()
M.x = Var()
M.p = Param(mutable=True)

e = M.p + M.x
s = set([type(M.p)])
assert list(EXPR.identify_components(e, s)) == [M.p]

The identify_variables function is a generator function that yields all nodes that are variables. Pyomo uses several different classes to represent variables, but this set of variable types does not need to be specified by the user. However, the include_fixed flag can be specified to omit fixed variables. For example:

import pyomo.core.expr as EXPR

M = ConcreteModel()
M.x = Var()
M.y = Var()

e = M.x + M.y
M.y.value = 1
M.y.fixed = True

assert set(id(v) for v in EXPR.identify_variables(e)) == set([id(M.x), id(M.y)])
assert set(id(v) for v in EXPR.identify_variables(e, include_fixed=False)) == set(
    [id(M.x)]
)

Walking an Expression Tree with a Visitor Class

Many of the utility functions defined above are implemented by walking an expression tree and performing an operation at nodes in the tree. For example, evaluating an expression is performed using a post-order depth-first search process where the value of a node is computed using the values of its children.

Walking an expression tree can be tricky, and the code requires intimate knowledge of the design of the expression system. Pyomo includes several classes that define visitor patterns for walking expression tree:

StreamBasedExpressionVisitor

The most general and extensible visitor class. This visitor implements an event-based approach for walking the tree inspired by the expat library for processing XML files. The visitor has seven event callbacks that users can hook into, providing very fine-grained control over the expression walker.

ExpressionValueVisitor

When the visitor() method is called on each node in the tree, the values of its children have been computed. The value of the node is returned from visitor().

ExpressionReplacementVisitor

When the visitor() method is called on each node in the tree, it may clone or otherwise replace the node using objects for its children (which themselves may be clones or replacements from the original child objects). The new node object is returned from visitor().

These classes define a variety of suitable tree search methods:

  • StreamBasedExpressionVisitor

    • walk_expression: depth-first traversal of the expression tree.

  • ExpressionReplacementVisitor

    • walk_expression: depth-first traversal of the expression tree.

  • ExpressionValueVisitor

    • dfs_postorder_stack: postorder depth-first search using a nonrecursive stack

To implement a visitor object, a user needs to provide specializations for specific events. For legacy visitors based on the PyUtilib visitor pattern (e.g., ExpressionValueVisitor), one must create a subclass and override at least one of the following:

visit()

Defines the operation that is performed when a node is visited. In the ExpressionValueVisitor and ExpressionReplacementVisitor visitor classes, this method returns a value that is used by its parent node.

visiting_potential_leaf()

Checks if the search should terminate with this node. If no, then this method returns the tuple (False, None). If yes, then this method returns (False, value), where value is computed by this method.

finalize()

This method defines the final value that is returned from the visitor. This is not normally redefined.

For modern visitors based on the StreamBasedExpressionVisitor, one can either define a subclass, pass the callbacks to an instance of the base class, or assign the callbacks as attributes on an instance of the base class. The StreamBasedExpressionVisitor provides seven callbacks, which are documented in the class documentation.

Detailed documentation of the APIs for these methods is provided with the class documentation for these visitors.

StreamBasedExpressionVisitor Example

In this example, we describe an visitor class that counts the number of nodes in an expression (including leaf nodes). Consider the following class:

import pyomo.core.expr as EXPR


class SizeofVisitor(EXPR.StreamBasedExpressionVisitor):
    def initializeWalker(self, expr):
        self.counter = 0
        return True, expr

    def exitNode(self, node, data):
        self.counter += 1

    def finalizeResult(self, result):
        return self.counter

The initializeWalker() method creates a counter, and the exitNode() method increments this counter for every node that is visited. The finalizeResult() method returns the value of this counter after the tree has been walked. The following function illustrates this use of this visitor class:

def sizeof_expression(expr):
    #
    # Create the visitor object
    #
    visitor = SizeofVisitor()
    #
    # Compute the value using the :func:`walk_expression` search method.
    #
    return visitor.walk_expression(expr)

ExpressionValueVisitor Example

In this example, we describe an visitor class that clones the expression tree (including leaf nodes). Consider the following class:

import pyomo.core.expr as EXPR


class CloneVisitor(EXPR.ExpressionValueVisitor):
    def __init__(self):
        self.memo = {'__block_scope__': {id(None): False}}

    def visit(self, node, values):
        #
        # Clone the interior node
        #
        return node.create_node_with_local_data(values)

    def visiting_potential_leaf(self, node):
        #
        # Clone leaf nodes in the expression tree
        #
        if node.__class__ in native_numeric_types or not node.is_expression_type():
            return True, copy.deepcopy(node, self.memo)

        return False, None

The visit() method creates a new expression node with children specified by values. The visiting_potential_leaf() method performs a deepcopy() on leaf nodes, which are native Python types or non-expression objects.

def clone_expression(expr):
    #
    # Create the visitor object
    #
    visitor = CloneVisitor()
    #
    # Clone the expression using the :func:`dfs_postorder_stack`
    # search method.
    #
    return visitor.dfs_postorder_stack(expr)

ExpressionReplacementVisitor Example

In this example, we describe an visitor class that replaces variables with scaled variables, using a mutable parameter that can be modified later. the following class:

import pyomo.core.expr as EXPR


class ScalingVisitor(EXPR.ExpressionReplacementVisitor):
    def __init__(self, scale):
        super(ScalingVisitor, self).__init__()
        self.scale = scale

    def beforeChild(self, node, child, child_idx):
        #
        # Native numeric types are terminal nodes; this also catches all
        # nodes that do not conform to the ExpressionBase API (i.e.,
        # define is_variable_type)
        #
        if child.__class__ in native_numeric_types:
            return False, child
        #
        # Replace leaf variables with scaled variables
        #
        if child.is_variable_type():
            return False, self.scale[id(child)] * child
        #
        # Everything else can be processed normally
        #
        return True, None

No other method need to be defined. The beforeChild() method identifies variable nodes and returns a product expression that contains a mutable parameter.

def scale_expression(expr, scale):
    #
    # Create the visitor object
    #
    visitor = ScalingVisitor(scale)
    #
    # Scale the expression using the :func:`dfs_postorder_stack`
    # search method.
    #
    return visitor.walk_expression(expr)

The scale_expression() function is called with an expression and a dictionary, scale, that maps variable ID to model parameter. For example:

M = ConcreteModel()
M.x = Var(range(5))
M.p = Param(range(5), mutable=True)

scale = {}
for i in M.x:
    scale[id(M.x[i])] = M.p[i]

e = quicksum(M.x[i] for i in M.x)
f = scale_expression(e, scale)

# p[0]*x[0] + p[1]*x[1] + p[2]*x[2] + p[3]*x[3] + p[4]*x[4]
print(f)