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
There are two dashes before
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
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
mip cuts all would be specified as
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
--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.
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
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
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
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
Here are some of the choices:
- lp: generate a standard linear programming format file with filename
- nlp: generate a file with a standard format that supports linear and
nonlinear optimization with filename extension
- os: generate an OSiL format XML file.
- python: use the direct Python interface.
Not all solvers support all interfaces.