CPMpy choco interface (cpmpy.solvers.choco)

Interface to Choco solver’s Python API.

Choco-solver is an open-source Java library for Constraint Programming (see https://choco-solver.org/). It comes with many features such as various types of variables, various state-of-the-art constraints, various search strategies, etc.

Always use cp.SolverLookup.get("choco") to instantiate the solver object.

Installation

Requires that the ‘pychoco’ python package is installed:

$ pip install pychoco

Detailed installation instructions available at:

The rest of this documentation is for advanced users.

List of classes

CPM_choco

Interface to the Choco solver python API

Module details

class cpmpy.solvers.choco.CPM_choco(cpm_model=None, subsolver=None)[source]

Interface to the Choco solver python API

Creates the following attributes (see parent constructor for more):

  • chc_model : the pychoco.Model() created by _model()

  • chc_solver : the choco Model().get_solver() instance used in solve()

Documentation of the solver’s own Python API:

add(cpm_expr)[source]

Eagerly add a constraint to the underlying solver.

Any CPMpy expression given is immediately transformed (through transform()) and then posted to the solver in this function.

This can raise ‘NotImplementedError’ for any constraint not supported after transformation

The variables used in expressions given to add are stored as ‘user variables’. Those are the only ones the user knows and cares about (and will be populated with a value after solve). All other variables are auxiliary variables created by transformations.

Parameters:

cpm_expr (Expression or list of Expression) – CPMpy expression, or list thereof

Returns:

self

get_core()

For use with s.solve(assumptions=[...]). Only meaningful if the solver returned UNSAT.

Typically implemented in SAT-based solvers

Returns a small subset of assumption literals that are unsat together. (a literal is either a _BoolVarImpl or a NegBoolView in case of its negation, e.g. x or ~x) Setting these literals to True makes the model UNSAT, setting any to False makes it SAT

has_objective()[source]

Returns whether the solver has an objective function or not.

maximize(expr)

Post the given expression to the solver as objective to maximize

maximize() can be called multiple times, only the last one is stored

minimize(expr)

Post the given expression to the solver as objective to minimize

minimize() can be called multiple times, only the last one is stored

property native_model

Returns the solver’s underlying native model (for direct solver access).

objective(expr, minimize)[source]

Post the given expression to the solver as objective to minimize/maximize

Parameters:

  • expr – Expression, the CPMpy expression that represents the objective function

  • minimize – Bool, whether it is a minimization problem (True) or maximization problem (False)

objective() can be called multiple times, only the last one is stored

Note

technical side note: constraints created during conversion of the objective are premanently posted to the solver. Choco accepts variables to maximize or minimize so it is needed to post constraints and create auxiliary variables

objective_value()

Returns the value of the objective function of the latest solver run on this model

Returns:

an integer or ‘None’ if it is not run, or a satisfaction problem

solution_hint(cpm_vars, vals)

For warmstarting the solver with a variable assignment

Typically implemented in SAT-based solvers

Parameters:

  • cpm_vars – list of CPMpy variables

  • vals – list of (corresponding) values for the variables

solve(time_limit=None, **kwargs)[source]

Call the Choco solver

Parameters:

  • time_limit (float, optional) – maximum solve time in seconds

  • kwargs – any keyword argument, sets parameters of solver object

solveAll(display=None, time_limit=None, solution_limit=None, call_from_model=False, **kwargs)[source]

Compute all (optimal) solutions, map them to CPMpy and optionally display the solutions.

Parameters:

  • display – either a list of CPMpy expressions, OR a callback function, called with the variables after value-mapping default/None: nothing displayed

  • solution_limit – stop after this many solutions (default: None)

  • time_limit (float, optional) – maximum solve time in seconds

Returns:

number of solutions found

solver_var(cpm_var)[source]

Creates solver variable for cpmpy variable or returns from cache if previously created

solver_vars(cpm_vars)

Like solver_var() but for arbitrary shaped lists/tensors

status()
static supported()[source]

Check for support in current system setup. Return True if the system has package installed or supports solver, else returns False.

Returns:

Solver support by current system setup.

Return type:

[bool]

transform(cpm_expr)[source]

Transform arbitrary CPMpy expressions to constraints the solver supports

Implemented through chaining multiple solver-independent transformation functions from the cpmpy/transformations/ directory.

See the Adding a new solver docs on readthedocs for more information.

Parameters:

cpm_expr (Expression or list of Expression) – CPMpy expression, or list thereof

Returns:

list of Expression