There are a number of cases within IDAES where a modeler may wish to use an Expression in place of a Var to reduce the complexity of their model. A common example of this is in the ideal Separator unit where the outlet Ports use Expressions for the state variable in order to reduce the number of variables (and thus constraints) in the model.
In these cases, it is possible that a user might mistake the Expression for a Var and attempt to use methods such as fix() on it. In order to provide the user with a useful error message informing them that this will not work, IDAES has created a derived VarLikeExpression component for these situations. This component derives directly from Pyomo’s Expression component and implements common methods associated with Vars which will return an error message informing the user that the component is an Expression, and a suggestion on how to proceed.
idaes.core.util.misc
VarLikeExpression
SimpleVarLikeExpression
IndexedVarLikeExpression
_GeneralVarLikeExpressionData