Adding terms to constraints

Some constraints are assembled from several sources: a power balance at a bus collects a term from every line and every generator touching it. Building an explicit sum per row would defeat the point, so terms are added into rows already declared.

from collections import namedtuple

import examodels as exa

Bus = namedtuple("Bus", "i pd gs")
Arc = namedtuple("Arc", "i bus")
Gen = namedtuple("Gen", "i bus")
bus = [Bus(0, 1.0, 0.1), Bus(1, 0.5, 0.0)]
arc = [Arc(0, 0), Arc(1, 1)]
gen = [Gen(0, 0)]

core = exa.Core()
vm = exa.add_var(core, len(bus), start=1.0)
p = exa.add_var(core, len(arc))
pg = exa.add_var(core, len(gen))

balance = exa.add_con(core, lambda b: b.pd + b.gs * vm[b.i]**2, over=bus)

exa.add_con(core, balance, lambda a: (a.bus, p[a.i]),  over=arc)
exa.add_con(core, balance, lambda g: (g.bus, -pg[g.i]), over=gen)

The first call declares one row per bus and returns a handle. Each later call takes that handle and a function returning (row, expression); the expression is added into that row. This mirrors the backend’s add_con / add_con! pair.

No rows are added by an augmentation — model.ncon is unchanged by the second and third calls. A block may be augmented any number of times, from index sets of different shapes.

An empty block can also be declared first and filled entirely afterwards:

N = 4
y = exa.add_var(core, N)

blk = exa.add_con(core, N, lcon=2.0, ucon=2.0)      # N rows, no terms yet
exa.add_con(core, blk, lambda i: (i, y[i]), over=range(N))