# The pyomo Command¶

The pyomo command is issued to the DOS prompt or a Unix shell. To see a list of Pyomo command line options, use:

pyomo solve --help


Note

There are two dashes before help.

In this section we will detail some of the options.

## Passing Options to a Solver¶

To pass arguments to a solver when using the pyomo solve command, appned the Pyomo command line with the argument --solver-options= followed by an argument that is a string to be sent to the solver (perhaps with dashes added by Pyomo). So for most MIP solvers, the mip gap can be set using

--solver-options= "mipgap=0.01 "


Multiple options are separated by a space. Options that do not take an argument should be specified with the equals sign followed by either a space or the end of the string.

For example, to specify that the solver is GLPK, then to specify a mipgap of two percent and the GLPK cuts option, use

solver=glpk --solver-options="mipgap=0.02 cuts="


If there are multiple “levels” to the keyword, as is the case for some Gurobi and CPLEX options, the tokens are separated by underscore. For example, mip cuts all would be specified as mip_cuts_all. For another example, to set the solver to be CPLEX, then to set a mip gap of one percent and to specify ‘y’ for the sub-option numerical to the option emphasis use

--solver=cplex --solver-options="mipgap=0.001 emphasis_numerical=y"


See Sending Options to the Solver for a discussion of passing options in a script.

## Troubleshooting¶

Many of things that can go wrong are covered by error messages, but sometimes they can be confusing or do not provide enough information. Depending on what the troubles are, there might be ways to get a little additional information.

If there are syntax errors in the model file, for example, it can occasionally be helpful to get error messages directly from the Python interpreter rather than through Pyomo. Suppose the name of the model file is scuc.py, then

python scuc.py


can sometimes give useful information for fixing syntax errors.

When there are no syntax errors, but there troubles reading the data or generating the information to pass to a solver, then the --verbose option provides a trace of the execution of Pyomo. The user should be aware that for some models this option can generate a lot of output.

If there are troubles with solver (i.e., after Pyomo has output “Applying Solver”), it is often helpful to use the option --stream-solver that causes the solver output to be displayed rather than trapped. (See <<TeeTrue>> for information about getting this output in a script). Advanced users may wish to examine the files that are generated to be passed to a solver. The type of file generated is controlled by the --solver-io option and the --keepfiles option instructs pyomo to keep the files and output their names. However, the --symbolic-solver-labels option should usually also be specified so that meaningful names are used in these files.

When there seem to be troubles expressing the model, it is often useful to embed print commands in the model in places that will yield helpful information. Consider the following snippet:

def ax_constraint_rule(model, i):
# return the expression for the constraint for i
print ("ax_constraint_rule was called for i=",str(i))
return sum(model.a[i,j] * model.x[j] for j in model.J) >= model.b[i]

# the next line creates one constraint for each member of the set model.I
model.AxbConstraint = Constraint(model.I, rule=ax_constraint_rule)


The effect will be to output every member of the set model.I at the time the constraint named model.AxbConstraint is constructed.

## Direct Interfaces to Solvers¶

In many applications, the default solver interface works well. However, in some cases it is useful to specify the interface using the solver-io option. For example, if the solver supports a direct Python interface, then the option would be specified on the command line as

--solver-io=python


Here are some of the choices:

• lp: generate a standard linear programming format file with filename extension lp
• nlp: generate a file with a standard format that supports linear and nonlinear optimization with filename extension n1lp
• os: generate an OSiL format XML file.
• python: use the direct Python interface.

Note

Not all solvers support all interfaces.