trees
¶
Tools for working with Boolean expression trees.
It should be noted that virtually all of the functionality within this module
is presented with an easiertouse interface in the expressions
module.
trees.tree_node
module¶
A node, and related classes, for use in expression trees.

class
tt.trees.tree_node.
BinaryOperatorExpressionTreeNode
(operator_str, l_child, r_child)[source]¶ Bases:
tt.trees.tree_node.ExpressionTreeNode
An expression tree node for binary operators.

operator
¶ The actual operator object wrapped in this node.
Type: BooleanOperator


class
tt.trees.tree_node.
ExpressionTreeNode
(symbol_name, l_child=None, r_child=None)[source]¶ Bases:
object
A base class for expression tree nodes.
This class is extended within tt and is not meant to be used directly.
If you plan to extend it, note that descendants of this class must compute the
_is_cnf
,_is_dnf
, and_is_really_unary
boolean attributes and the_non_negated_symbol_set
and_negated_symbol_set
set attributes within their initialization. Additionally, descendants of this class must implemented the__eq__
magic method (but not__ne__
) as well as the private_copy
transformation.
apply_de_morgans
()[source]¶ Return a transformed node, with De Morgan’s Law applied.
Since nodes are immutable, the returned node, and all descendants, are new objects.
Returns: An expression tree node with all negated AND and OR operators transformed, following De Morgan’s Law. Return type: ExpressionTreeNode

apply_idempotent_law
()[source]¶ Returns a transformed node, with the Idempotent Law applied.
Since nodes are immutable, the returned node, and all descendants, are new objects
Returns: An expression tree node with the Idempotent Law applied to AND and OR operators. Return type: ExpressionTreeNode
This transformation will apply the Idempotent Law to AND and OR expressions involving repeated operands. Here are a few examples:
>>> from tt import BooleanExpression >>> tree = BooleanExpression('A and A').tree >>> print(tree.apply_idempotent_law()) A >>> tree = BooleanExpression('~B or ~~~B').tree >>> print(tree.apply_idempotent_law()) ~ `B
In the latter of the two above examples, we see that this transformation will compare operands with negations condensed. This transformation will also prune redundant operands from CNF and DNF clauses. Let’s take a look:
>>> from tt import BooleanExpression >>> tree = BooleanExpression('A and B and B and C and ~C and ~~C and D').tree >>> print(tree.apply_idempotent_law()) and `and  `and   `and    `A    `B   `C  `~  `C `D

apply_identity_law
()[source]¶ Return a transformed node, with the Identity Law applied.
Since nodes are immutable, the returned node, and all descendants, are new objects.
This transformation will achieve the following effects by applying the Inverse Law to the AND and OR operators:
>>> from tt import BooleanExpression >>> tree = BooleanExpression('A and 1').tree >>> print(tree.apply_identity_law()) A >>> tree = BooleanExpression('0 or B').tree >>> print(tree.apply_identity_law()) B
It should also be noted that this transformation will also apply the annihilator properties of the logical AND and OR operators. For example:
>>> from tt import BooleanExpression >>> tree = BooleanExpression('A and 0').tree >>> print(tree.apply_identity_law()) 0 >>> tree = BooleanExpression('1 or B').tree >>> print(tree.apply_identity_law()) 1
Returns: An expression tree node with AND and OR identities simplified. Return type: ExpressionTreeNode

apply_inverse_law
()[source]¶ Return a transformed node, with the Inverse Law applied.
Since nodes are immutable, the returned node, and all descendants, are new objects.
Returns: An expression tree node with the Inverse Law applied to applicable clauses. Return type: ExpressionTreeNode
This transformation will apply the Inverse Law to AND and OR expressions involving the negated and nonnegated forms of a variable. Here are a few examples:
>>> from tt import BooleanExpression >>> tree = BooleanExpression('~A and A').tree >>> print(tree.apply_inverse_law()) 0 >>> tree = BooleanExpression('B or !B').tree >>> print(tree.apply_inverse_law()) 1
Note that this transformation will not reduce expressions of constants; the transformation
apply_identity_law
will probably do what you want in this case, though.This transformation will also reduce expressions in CNF or DNF that contain negated and nonnegated forms of the same symbol. Let’s take a look:
>>> from tt import BooleanExpression >>> tree = BooleanExpression('A or B or C or ~B').tree >>> print(tree.apply_inverse_law()) 1 >>> tree = BooleanExpression('A and B and C and !B').tree >>> print(tree.apply_inverse_law()) 0

static
build_tree
(postfix_tokens)[source]¶ Build a tree from a list of expression tokens in postfix order.
This method does not check that the tokens are indeed in postfix order; undefined behavior will ensue if you pass tokens in an order other than postfix.
Parameters: postfix_tokens (List[
str
]) – A list of string tokens from which to construct the tree of expression nodes.Returns: The root node of the constructed tree.
Return type: Raises:  InvalidArgumentTypeError – If
postfix_tokens
is not a list of strings.  InvalidArgumentValueError – If
postfix_tokens
is empty.
 InvalidArgumentTypeError – If

coalesce_negations
()[source]¶ Return a transformed node, with consecutive negations coalesced.
Since nodes are immutable, the returned node, and all descendants, are new objects.
Returns: An expression tree node with all consecutive negations compressed into the minimal number of equivalent negations (either one or none). Return type: ExpressionTreeNode

distribute_ands
()[source]¶ Return a transformed nodes, with ANDs recursively distributed across ORed subexpressions.
Since nodes are immutable, the returned node, and all descendants, are new objects.
Returns: An expression tree node with all applicable AND operators distributed across ORed subexpressions. Return type: ExpressionTreeNode

distribute_ors
()[source]¶ Return a transformed nodes, with ORs recursively distributed across ANDed subexpressions.
Since nodes are immutable, the returned node, and all descendants, are new objects.
Returns: An expression tree node with all applicable OR operators distributed across ANDed subexpressions. Return type: ExpressionTreeNode

evaluate
(input_dict)[source]¶ Recursively evaluate this node.
This is an interface that should be defined in subclasses. Node evaluation does no checking of the validity of inputs; they should be check before being passed here.
Parameters: input_dict (Dict{ str
: truthy) – A dictionary mapping expression symbols to the value for which they should be subsituted in expression evaluation.Returns: The evaluation of the tree rooted at this node. Return type: bool

iter_clauses
()[source]¶ Iterate the clauses in the expression tree rooted at this node.
If the normal form of the expression is ambiguous, then precedence will be given to conjunctive normal form.
Returns: Iterator of each CNF or DNF clause, rooted by a tree node, contained within the expression tree rooted at this node. Return type: Iterator[ ExpressionTreeNode
]Raises: RequiresNormalFormError – If this expression is not in conjunctive or disjunctive normal form.

iter_cnf_clauses
()[source]¶ Iterate the clauses in conjunctive normal form order.
Returns: Iterator of each CNF clause, rooted by a tree node, contained within the expression tree rooted at this node. Return type: Iterator[ ExpressionTreeNode
]Raises: RequiresNormalFormError – If the expression tree rooted at this node is not in conjunctive normal form.

iter_dnf_clauses
()[source]¶ Iterate the clauses in disjunctive normal form order.
Returns: Iterator of each DNF clause, rooted by a tree node, contained within the expression tree rooted at this node. Return type: Iterator[ ExpressionTreeNode
]Raises: RequiresNormalFormError – If the expression tree rooted at this node is not in disjunctive normal form.

l_child
¶ This node’s left child;
None
indicates the absence of a child.Type: ExpressionTreeNode
orNone

non_negated_symbol_set
¶ A set of the nonnegated symbols present in the tree rooted here.
Type: Set[ str
]

r_child
¶ This node’s left child;
None
indicates the absence of a child.Type: ExpressionTreeNode
orNone

to_cnf
()[source]¶ Return a transformed node, in conjunctive normal form.
Since nodes are immutable, the returned node, and all descendants, are new objects.
Returns: An expression tree node with all operators transformed to consist only of NOTs, ANDs, and ORs. Return type: ExpressionTreeNode

to_primitives
()[source]¶ Return a transformed node, containing only NOTs, ANDs, and ORs.
Since nodes are immutable, the returned node, and all descendants, are new objects.
Returns: An expression tree node with all operators transformed to consist only of NOTs, ANDs, and ORs. Return type: ExpressionTreeNode


class
tt.trees.tree_node.
OperandExpressionTreeNode
(operand_str)[source]¶ Bases:
tt.trees.tree_node.ExpressionTreeNode
An expression tree node for operands.
Nodes of this type will always be leaves in an expression tree.

class
tt.trees.tree_node.
UnaryOperatorExpressionTreeNode
(operator_str, l_child)[source]¶ Bases:
tt.trees.tree_node.ExpressionTreeNode
An expression tree node for unary operators.

operator
¶ The actual operator object wrapped in this node.
Type: BooleanOperator
