Choosing a solver¶

How to solve on a solver other than HiGHS, pass it options, or write the model to a file for a solver specsolve does not run. HiGHS ships with specsolve and is the default. Nothing in the spec names a solver, so the call chooses one.

See which solvers this build has¶

Name a solver that does not exist, and the refusal lists the ones that do. This block runs when the site is built, so the list is this commit's:

import specsolve as sps

try:
    sps.solve('examples/dispatch.yaml', {}, solver_name='cplex')
except sps.SpecsolveError as exc:
    print(exc)

unknown solver 'cplex' — this build solves with gurobi, highs, xpress.

The name is checked before any data is attached.

Install the solver¶

Every solver except HiGHS comes with the extra of the same name:

pip install "specsolve[gurobi]"

A name the build knows, but whose package is not installed, raises ModuleNotFoundError with the pip install line to run.

Name it in the call¶

solver_name= goes on solve, Model.solve and solve_over:

result = sps.solve('dispatch.yaml', sources, solver_name='gurobi')

Pass solver options¶

solver_options= goes to the solver verbatim, in its own vocabulary. A time limit is time_limit on HiGHS and TimeLimit on Gurobi:

result = sps.solve('dispatch.yaml', sources, solver_name='gurobi', solver_options={'TimeLimit': 60})

Check a spec against a solver before solving¶

Solvers take different constructs: HiGHS has no SOS sets, for example. check with sink= refuses a spec the solver cannot take, and needs no data:

sps.check('dispatch.yaml', sink='highs')

The refusal names the solvers that do take the construct. The full table is what each sink takes.

Write the model to a file¶

For a solver or tool that specsolve does not run, write the model. The suffix picks the format, and this build writes .lp and .mps:

sps.write('dispatch.yaml', sources, 'dispatch.lp')

The two formats carry different constructs: MPS has no quadratic objective or indicator, for example. The table above covers both.

A written column is x0 and a row is c0 unless you ask for names. With names=True, each column and row is named by its declaration and coordinate, such as p(0,wind) and balance(0). Use it when the tool that reads the file finds a variable by name, as JuMP's variable_by_name does:

sps.write('dispatch.yaml', sources, 'dispatch.lp', names=True)

A label keeps its letters, digits and !"#$%&'.;?@`{|}~. Every other character is written as _, because an LP reader refuses it, so north sea becomes north_sea. A declaration with no dims is written total(). When two labels of one declaration become the same name, the write is refused and the error names both labels.