Python API - Constr#

class Constr#

Gurobi constraint object. Constraints are always associated with a particular model. You create a constraint object by adding a constraint to a model (using Model.addConstr), rather than by using a Constr constructor.

Constraint objects have a number of attributes. The full list can be found in the Attributes section of this document. Some constraint attributes can only be queried, while others can also be set. Recall that the Gurobi Optimizer employs a lazy update approach, so changes to attributes don’t take effect until the next call to Model.update, Model.optimize, or Model.write on the associated model.

We should point out a few things about constraint attributes. Consider the rhs attribute. Its value can be queried using constr.rhs. The Gurobi library ignores letter case in attribute names, so it can also be queried as constr.rhs. It can be set using a standard assignment statement (e.g., constr.rhs = 0). However, as mentioned earlier, attribute modification is done in a lazy fashion, so you won’t see the effect of the change immediately. And some attributes can not be set (e.g., the Pi attribute), so attempts to assign new values to them will raise an exception.

You can also use Constr.getAttr/ Constr.setAttr to access attributes. The attribute name can be passed to these routines as a string, or you can use the constants defined in the GRB.Attr class (e.g., GRB.Attr.RHS).

getAttr(attrname)#

Query the value of a constraint attribute. The full list of available attributes can be found in the Attributes section.

Raises an AttributeError if the requested attribute doesn’t exist or can’t be queried. Raises a GurobiError if there is a problem with the Constr object (e.g., it was removed from the model).

Parameters:

attrname – The attribute being queried.

Returns:

The current value of the requested attribute.

Example:
print(constr.getAttr(GRB.Attr.Slack))
print(constr.getAttr("slack"))
property index#

This property returns the current index, or order, of the constraint in the underlying constraint matrix.

Note that the index of a constraint may change after subsequent model modifications.

Returns:

-2: removed, -1: not in model, otherwise: index of the constraint in the model

Example:
c = model.getConstrs()[0]
print(c.index) # Index will be 0
sameAs(constr2)#

Check whether two constraint objects refer to the same constraint.

Parameters:

constr2 – The other constraint.

Returns:

Boolean result indicates whether the two constraint objects refer to the same model constraint.

Example:
print(model.getConstrs()[0].sameAs(model.getConstrs()[1]))
setAttr(attrname, newvalue)#

Set the value of a constraint attribute. Note that, due to our lazy update approach, the change won’t actually take effect until you update the model (using Model.update), optimize the model (using Model.optimize), or write the model to disk (using Model.write).

The full list of available attributes can be found in the Attributes section.

Raises an AttributeError if the specified attribute doesn’t exist or can’t be set. Raises a GurobiError if there is a problem with the Constr object (e.g., it was removed from the model).

Parameters:

  • attrname – The attribute being modified.

  • newvalue – The desired new value of the attribute.

Example:
constr.setAttr(GRB.Attr.RHS, 0.0)
constr.setAttr("rhs", 0.0)