# Constraints

Most constraints are specified using equality or inequality expressions that are created using a rule, which is a Python function. For example, if the variable model.x has the indexes ‘butter’ and ‘scones’, then this constraint limits the sum over these indexes to be exactly three:

def teaOKrule(model):
return(model.x['butter'] + model.x['scones'] == 3)
model.TeaConst = Constraint(rule=teaOKrule)


Instead of expressions involving equality (==) or inequalities (<= or >=), constraints can also be expressed using a 3-tuple if the form (lb, expr, ub) where lb and ub can be None, which is interpreted as lb <= expr <= ub. Variables can appear only in the middle expr. For example, the following two constraint declarations have the same meaning:

model.x = Var()

def aRule(model):
return model.x >= 2
model.Boundx = Constraint(rule=aRule)

def bRule(model):
return (2, model.x, None)
model.boundx = Constraint(rule=bRule)


For this simple example, it would also be possible to declare model.x with a bounds option to accomplish the same thing.

Constraints (and objectives) can be indexed by lists or sets. When the declaration contains lists or sets as arguments, the elements are iteratively passed to the rule function. If there is more than one, then the cross product is sent. For example the following constraint could be interpreted as placing a budget of $$i$$ on the $$i^{\mbox{th}}$$ item to buy where the cost per item is given by the parameter model.a:

model.A = RangeSet(1,10)
model.a = Param(model.A, within=PositiveReals)
model.ToBuy = Var(model.A)
def bud_rule(model, i):
return model.a[i]*model.ToBuy[i] <= i
aBudget = Constraint(model.A, rule=bud_rule)


Note

Python and Pyomo are case sensitive so model.a is not the same as model.A.