pybnb.problem

Branch-and-bound problem definition.

Copyright by Gabriel A. Hackebeil (gabe.hackebeil@gmail.com).

class pybnb.problem.Problem

The abstract base class used for defining branch-and-bound problems.

abstractmethod sense()

Returns the objective sense for this problem.

Note

This method is abstract and must be defined by the user.

abstractmethod bound()

Returns a value that is a bound on the objective of the current problem state or self.unbounded_objective() if no non-trivial bound is available.

Note

This method is abstract and must be defined by the user.

abstractmethod objective()

Returns a feasible value for the objective of the current problem state or self.infeasible_objective() if the current state is not feasible.

Note

This method is abstract and must be defined by the user.

abstractmethod save_state(node)

Saves the current problem state into the given pybnb.node.Node object.

This method is guaranteed to be called once at the start of the solve by all processes involved to collect the root node problem state, but it may be called additional times. When it is called for the root node, the node.tree_depth will be zero.

Note

This method is abstract and must be defined by the user.

abstractmethod load_state(node)

Loads the problem state that is stored on the given pybnb.node.Node object.

Note

This method is abstract and must be defined by the user.

abstractmethod branch()

Returns a list of Node objects that partition the node state into zero or more children. This method can also be defined as a generator.

Note

This method is abstract and must be defined by the user.

infeasible_objective()

Returns the value that represents an infeasible objective (i.e., +inf or -inf depending on the sense). The Problem base class implements this method.

unbounded_objective()

Returns the value that represents an unbounded objective (i.e., +inf or -inf depending on the sense). The Problem base class implements this method.

notify_solve_begins(comm, worker_comm, convergence_checker)

Called when a branch-and-bound solver begins as solve. The Problem base class provides a default implementation for this method that does nothing.

Parameters

• comm (mpi4py.MPI.Comm or None) – The full MPI communicator that includes all processes. Will be None if MPI has been disabled.

• worker_comm (mpi4py.MPI.Comm or None) – The MPI communicator that includes only worker processes. Will be None if MPI has been disabled.

• convergence_checker (ConvergenceChecker:) – The class used for comparing the objective and bound values during the solve.

notify_new_best_node(node, current)

Called when a branch-and-bound solver receives a new best node from the dispatcher. The Problem base class provides a default implementation for this method that does nothing.

Parameters

• node (Node) – The new best node.

• current (bool) – Indicates whether or not the node argument is the currently loaded node (from the most recent load_state call).

notify_solve_finished(comm, worker_comm, results)

Called when a branch-and-bound solver finishes. The Problem base class provides a default implementation for this method that does nothing.

Parameters

• comm (mpi4py.MPI.Comm or None) – The full MPI communicator that includes all processes. Will be None if MPI has been disabled.

• worker_comm (mpi4py.MPI.Comm or None) – The MPI communicator that includes only worker processes. Will be None if MPI has been disabled.

• results (SolverResults) – The fully populated results container that will be returned from the solver.