Unexpected lack of input validation for Pyomo slices #4051

Summary

I spent a while troubleshooting a bug in a PrOMMiS model. The problem is that a variable was being fixed for an invalid index.

            # Should be fixing it for "H_+"
            self.leach.liquid_inlet.conc_mass_comp[:, "H"].fix(
                2 * 0.05 * 1e3 * units.mg / units.L
            )

Using the wrong index was my own fault, but I'd expect Pyomo input validation to recognize that the index is invalid and return a KeyError.

Steps to reproduce the issue

# example.py
import pyomo.environ as pyo

m = pyo.ConcreteModel()
m.x = pyo.Var(["a", "b", "c"], [1, 2, 3])

m.x["d", :].fix(0)

Error Message

I'd expect a KeyError to be raised, because "d" isn't a valid index for the first index. Instead, it continues, returning an empty list. Trying to fix the slice along a valid index returns a list with three Nones.

Information on your system

Pyomo version: 6.10.1
Python version: 3.11
Operating system: Windows
How Pyomo was installed (PyPI, conda, source): PrOMMiS dev installation
Solver (if applicable): n/a