Connector features

[eslickj]

Connectors would be more useful with a few additional features

1. It would be helpful if constraints that connect two connectors were easier to identify without looking at the variables in every constraint and checking if they are connectors. Maybe connections could be their own special subclass of constraint.
2. Once the connectors are expanded, it would be also nice if the resulting constraints could be related back to the connection in some way and could be easily activated or deactivated as a group like connection.deactivate() and that would deactivate the constraints that result from the expanded constraint.
3. It would also be nice to be able to deactivate just a particular constraint in a connection. Like if connectors had T and P in them, you could do something like connection.deactivate("T")
4. Slicing would be nice also Connectors do not support sliced variables #227

[qtothec]

I'm not sure what the impact on performance would be, but core.expand_connectors could create a ComponentMap linking the connector constraints to a block where their expanded form resides, similar to how the GDP transformation works. Deactivating that block would thus provide the functionality in (2). I think that pulling (3) off would be a good bit more difficult, especially working around @jsiirola's aggregators.

[andrewlee94]

Another feature that I have found myself wishing for is the ability to fix and unfix an associated Var through a Connector. Whilst I can always go and fix the Var in its parent component, sometimes it would be more convenient to do so through the Connector. In many cases, the Connector is the most obvious outward facing component of the model, rather than the associatedVar which might be buried deeper in the model structure.

[qtothec]

@andrewlee94 To clarify, are you looking to simultaneously fix all of the Var objects associated with a particular Connector? You are able to access the vars associated with a connector already.

[andrewlee94]

This may be more specific to indexed connectors, but I would like to be able to write something like model.connector[x].var_1.fix(...). Currently if I try that I get an AttributeError saying:
'_ConnectorData' object has no attribute 'var_1'

[qtothec]

The connector data object has an attribute called vars. Try that.

[andrewlee94]

@qtothec Well there you go, thank you for pointing that out. I had thought it was an obvious feature to have.

[jsiirola]

There's a lot of good ideas here. Connectors are an old feature and are (over)due for a revamp. Can we carve out some time next week for the 3 of us to break out and write down the use cases - both the ones we are using now and the new features above?

[qtothec]

Yes, I think that would be a good plan. Connectors are pretty high up my list of agenda items. I'm currently working on a prioritized list of software framework issues.

[jsiirola]

Fantastic.

[blnicho]

Another use case that should be discussed is adding ContinuousSet indexed variables to a Connector. I have some ideas on how to "re-expand" an already expanded Connector when applying a discretization transformation but if there is a way to incorporate this into a Connector re-design I think it would be a better long-term solution.

[eslickj]

I don't think we ever settled the connector design at the January meeting. I would probably be good to follow up on this soon.

[blnicho]

Closing this as most of it was resolved in #583. New issues should be opened for any feature requests in pyomo.network.