Model

The model class is used to collect equations into groups and to label them so that they can be solved. It also allows specifying the problem type (e.g. LP, MIP etc.), sense of the problem (MIN, MAX, FEASIBILITY) and the objective of the problem at hand.

The overall syntax of a model is as follows:

from gamspy import Container, Variable, Equation, Model, Sense, Problem

m = Container()

z = Variable(m, "z") # objective variable
e1 = Equation(m, "e1")
e1[...] = <definition_of_the_equation>
e2 = Equation(m, "e2")
e2[...] = <definition_of_the_equation>

example_model = Model(m, "dummy", equations=[e1,e2], problem=Problem.LP, sense=Sense.Max, objective=z)

Classification of Models

Various types of problems can be solved with GAMS. Note that the type of the model must be known before it may be solved. The model types are briefly discussed in this section. GAMS checks that the model is in fact the type the user thinks it is, and issues explanatory error messages if it discovers a mismatch - for instance, that a supposedly linear model contains nonlinear terms. Some problems may be solved in more than one way, and the user has to choose which way to use. For instance, if there are binary or integer variables in the model, it can be solved either as a MIP or as a RMIP.

Model Type	Model Type Description
LP	Linear Program
NLP	Nonlinear Program
QCP	Quadratically Constrained Program
DNLP	Discontinuous Nonlinear Program
MIP	Mixed Integer Program
RMIP	Relaxed Mixed Integer Program
MINLP	Mixed Integer Nonlinear Program
RMINLP	Relaxed Mixed Integer Nonlinear Program
MIQCP	Mixed Integer Quadratically Constrained Program
RMIQCP	Relaxed Mixed Integer Quadratically Constrained Program
MCP	Mixed Complementarity Problem
CNS	Constrained Nonlinear System
MPEC	Mathematical Programs with Equilibrium Constraints
RMPEC	Relaxed Mathematical Program with Equilibrium Constraints
EMP	Extended Mathematical Program
MPSGE	General Equilibrium

All model types are exposed with Problem enum but the problem type can be specified as a string as well.

Also the direction types of the optimization (MIN, MAX, or FEASIBILITY) are exposed with Sense enum but it can be specified as a string similarly.

Model Attributes

Models have attributes that hold a variety of information, including

• information about the results of a solve performed, a solve statement, the solution of a model,

• information about certain features to be used by GAMS or the solver,

• information passed to GAMS or the solver specifying various settings that are also subject to option statements.

Model Attribute	Description
num_domain_violations	Number of domain violations
algorithm_time	Solver dependent timing information
total_solve_time	Elapsed time it took to execute a solve statement in total
total_solver_time	Elapsed time taken by the solver only
num_iterations	Number of iterations used
marginals	Indicator for marginals present
max_infeasibility	Maximum of infeasibilities
mean_infeasibility	Mean of infeasibilities
status	Integer number that indicates the model status
num_nodes_used	Number of nodes used by the MIP solver
num_dependencies	Number of dependencies in a CNS model
num_discrete_variables	Number of discrete variables
num_infeasibilities	Number of infeasibilities
num_nonlinear_insts	Number of nonlinear instructions
num_nonlinear_zeros	Number of nonlinear nonzeros
num_nonoptimalities	Number of nonoptimalities
num_nonzeros	Number of nonzero entries in the model coefficient matrix
num_mcp_redefinitions	Number of MCP redefinitions
num_variables	Number of variables
num_bound_projections	Number of bound projections during model generation
objective_estimation	Estimate of the best possible solution for a mixed-integer model
objective_value	Objective function value
used_model_type	Integer number that indicates the used model type
model_generation_time	Time GAMS took to generate the model in wall-clock seconds
solve_model_time	Time the solver used to solve the model in seconds
sum_infeasibilities	Sum of infeasibilities
solver_status	Indicates the solver termination condition
solver_version	Solver version

Solving a Model

Model has a function named solve that allows user to solve the specified model.

from gamspy import Container, Variable, Equation, Model, Sense, Problem, Options

m = Container()

z = Variable(m, "z") # objective variable
e1 = Equation(m, "e1")
e1[...] = <definition_of_the_equation>
e2 = Equation(m, "e2")
e2[...] = <definition_of_the_equation>

model = Model(m, "dummy", equations=[e1,e2], problem=Problem.LP, sense=Sense.Max, objective=z)
model.solve(solver="CONOPT", options=Options(iteration_limit=2), solver_options={"rtmaxv": "1.e12"})

In most cases, calling the solve function of your model without any parameters is sufficient. In this scenario, the default solver depending on the problem type, default options will be used. But for users who requires a higher level of control can set the solver to be used, general options and solver specific options. All installed solvers on your system can be queried by running the following command:

gamspy list solvers

If you want to get all available solvers that you can install and use, the following command would give you the list of solvers that are available.:

gamspy list solvers -a

Solve Options

Solve options can be specified as an gamspy.Options() class. For example:

from gamspy import Container, Variable, Equation, Model, Sense, Problem, Options

m = Container()

...
...
Definition of your model
...
...

model = Model(m, "my_model", equations=m.getEquations(), problem=Problem.LP, sense=Sense.Max, objective=z)
model.solve(options=Options(iteration_limit=2))

Here is the list of options and their descriptions:

Option	Description	Possible Values
cns	Default cns solver	Any solver installed in your system that can solve cns
dnlp	Default dnlp solver	Any solver installed in your system that can solve dnlp
emp	Default emp solver	Any solver installed in your system that can solve emp
lp	Default lp solver	Any solver installed in your system that can solve lp
mcp	Default mcp solver	Any solver installed in your system that can solve mcp
minlp	Default minlp solver	Any solver installed in your system that can solve minlp
mip	Default mip solver	Any solver installed in your system that can solve mip
miqcp	Default miqcp solver	Any solver installed in your system that can solve miqcp
mpec	Default mpec solver	Any solver installed in your system that can solve mpec
nlp	Default nlp solver	Any solver installed in your system that can solve nlp
qcp	Default qcp solver	Any solver installed in your system that can solve qcp
rminlp	Default rminlp solver
rmip	Default rmip solver	Any solver installed in your system that can solve rmip
rmiqcp	Default rmiqcp solver
rmpec	Default rmpec solver	Any solver installed in your system that can solve rmpec
allow_suffix_in_equation	Allow variables with suffixes in model algebra	bool
allow_suffix_in_limited_variables	Allow domain limited variables with suffixes in model	bool
basis_detection_threshold	Basis detection threshold	float
compile_error_limit	Compile time error limiy	int
domain_violation_limit	Domain violation limit solver default	int
job_time_limit	Elapsed time limit in seconds	float
job_heap_limit	Maximum Heap size allowed in MB	float
hold_fixed_variables	Treat fixed variables as constants	bool
integer_variable_upper_bound	Set mode for default upper bounds on integer variables	0: Set to +INF 1: Set to 100. 2: Set to 100 and write to the log if the level > 100 3: Same as 2 but issues an error if the level > 100
iteration_limit	Iteration limit of solver	int
keep_temporary_files	Controls keeping or deletion of process directory and scratch files	bool
license	Use alternative license file	Path to the alternative license
variable_listing_limit	Maximum number of columns listed in one variable block	int
equation_listing_limit	Maximum number of rows listed in one equation block	int
node_limit	Node limit in branch and bound tree	int
absolute_optimality_gap	Absolute Optimality criterion solver default	float
relative_optimality_gap	Relative Optimality criterion solver default	float
profile	Execution profiling	0: No profiling 1: Minimum profiling 2: Profiling depth for nested control structures
profile_tolerance	Minimum time a statement must use to appear in profile generated output	float
time_limit	Wall-clock time limit for solver	float
savepoint	Save solver point in GDX file	0: No point GDX file is to be saved 1: A point GDX file from the last solve is to be saved 2: A point GDX file from every solve is to be saved 3: A point GDX file from the last solve is to be saved 4: A point GDX file from every solve is to be saved
seed	Random number seed	int
report_solution	Solution report print option	0: Remove solution listings following solves 1: Include solution listings following solves 2: Suppress all solution information
show_os_memory		0: Show memory reported by internal accounting 1: Show resident set size reported by operating system 2: Show virtual set size reported by operating system
solver_link_type	Solver link option	https://gams.com/45/docs/UG_GamsCall.html#GAMSAOsolvelink
multi_solve_strategy	Multiple solve management	“replace” | “merge” | “clear”
step_summary	Summary of computing resources used by job steps	bool
suppress_compiler_listing	Compiler listing option	bool
report_solver_status	Solver Status file reporting option	bool
threads	Number of threads to be used by a solver	int
trace_file	Trace file name	Name of the trace file
trace_level	Modelstat/Solvestat threshold used in conjunction with action=GT	int
trace_file_format	Trace file format option	0: Solver and GAMS step trace 1: Solver and GAMS exit trace 2: Solver trace only 3: Trace only in format used for GAMS performance world 5: Gams exit trace with all available trace fields
write_listing_file	Controls listing file creation	bool
zero_rounding_threshold	The results of certain operations will be set to zero if abs(result) LE ZeroRes	float
report_underflow	Report underflow as a warning when abs(results) LE ZeroRes and result set to zero	bool

Solver Options

In addition to solve options, user can specify solver options to be used by the solver as a dictionary. For all possible solver options, please check the corresponding solver manual

Solve Locally

Models are solved locally (on your machine) by default.

Solve Using GAMS Engine

In order to send your model to be solved to GAMS Engine, you need to define the configuration of GAMS Engine. This can be done by importing EngineConfig and creating an instance. Then, the user can pass it to the solve method and specify the backend as engine.

from gamspy import Container, Variable, Equation, Model, Sense, Problem

m = Container()

z = Variable(m, "z") # objective variable
e1 = Equation(m, "e1")
e1[...] = <definition_of_the_equation>
e2 = Equation(m, "e2")
e2[...] = <definition_of_the_equation>

model = Model(m, "dummy", equations=[e1,e2], problem=Problem.LP, sense=Sense.Max, objective=z)

config = EngineConfig(
    host=os.environ["ENGINE_URL"],
    username=os.environ["ENGINE_USER"],
    password=os.environ["ENGINE_PASSWORD"],
    namespace=os.environ["ENGINE_NAMESPACE"],
)
model.solve(solver="CONOPT", backend="engine", engine_config=config)

Redirecting Output

The output of GAMS after solving the model can be redirected to a file or to standard input by specifying the output parameter of the solve.:

from gamspy import Container, Variable, Equation, Model, Sense, Problem
import sys

m = Container()

z = Variable(m, "z") # objective variable
e1 = Equation(m, "e1")
e1[...] = <definition_of_the_equation>
e2 = Equation(m, "e2")
e2[...] = <definition_of_the_equation>

model = Model(m, "dummy", equations=[e1,e2], problem=Problem.LP, sense=Sense.Max, objective=z)

# redirect output to stdout
model.solve(output=sys.stdout)

# redirect output to a file
with open("my_out_file", "w") as file:
    model.solve(output=file)