Execution Modes and Debugging

Execution Modes

GAMSPy supports two execution modes through the Container.

Normal Execution

By default, normal execution is enabled. In this mode, each assignment or record reading attempt triggers the execution of generated code, and the results are saved. This mode is suitable for debugging, although it may be slower than delayed execution.

from gamspy import Container, Set, Parameter
m = Container()
i = Set(m, "i", records=["i1", "i2"])
a = Parameter(m, "a", domain=[i], records=[("i1", 1), ("i2", 2)])
a[i] = a[i] * 90

In normal execution, the last line executes the actual computation in GAMS, as soon as it’s specified.

Delayed Execution

Delayed execution is a mode designed for better performance. Assignments are not executed until the solve function of a model is called or an attempt is made to read the records of a dirty symbol. A dirty symbol is a symbol that was assigned a new value in previous lines.

from gamspy import Container, Set, Parameter
m = Container(delayed_execution=True)
i = Set(m, "i", records=["i1", "i2"])
a = Parameter(m, "a", domain=[i], records=[("i1", 1), ("i2", 2)])
a[i] = a[i] * 90 # This line is not executed yet. a is dirty now.
print(a.records) # An attempt to read a dirty symbol cause a GAMS run. a is not dirty anymore.

This behaviour allows GAMSPy to minimize the number of actual runs in the backend. The performance improvement compared to normal execution mode might differ depending the model significantly.

If you are familiar with GAMS language, and want to see the generated .gms file or .lst file, you can specify the working directory of the Container.

Debugging

Specifying the Working Directory

from gamspy import Container
m = Container(working_directory=".")
....
....
....
specify your model here
....
....
....
model.solve()

In this example, specifying the working directory as the current directory causes temporary GAMS files (.gms, .lst, .g00. ,gdx files etc.) to be saved in the current directory.

Generating a Log File

The output of GAMS can be redirected to standard output or to a file by specifying the handle for the destination. For example:

import sys
from gamspy import Container
m = Container(working_directory=".")
....
....
....
specify your model here
....
....
....
model.solve(output=sys.stdout)

The code snippet above redirects the GAMS execution output to your console by specifying the output as standard output. You can also redirect the output to a file:

import sys
from gamspy import Container
m = Container(working_directory=".")
....
....
....
specify your model here
....
....
....
with open("mylog.txt", "w") as log:
    model.solve(output=log)

This code snippets redirects the output of the execution to a file named “mylog.txt”.

If you want to have your log file generated in the working directory, create_log_file argument can be provided.

import sys
from gamspy import Container
m = Container(working_directory=".")
....
....
....
specify your model here
....
....
....
model.solve(create_log_file=True)

This code snippet would generate a log file in the specified working directory. This argument is also useful for both redirecting the output to standard output and generating the log file at the same time.

import sys
from gamspy import Container
m = Container(working_directory=".")
....
....
....
specify your model here
....
....
....
model.solve(output=sys.stdout, create_log_file=True)

This code snippet would redirect the output to your console as well as saving the log file in your working directory.

Inspecting Generated GAMS String

Another alternative is to use the generateGamsString function. This function returns the GAMS code generated up to that point as a string. This function must be used with delayed_execution mode, otherwise you will almost always get an empty string since the previous statements already ran with GAMS and the results were loaded into the container.

from gamspy import Container
m = Container(delayed_execution=True, working_directory=".")
....
....
....
print(m.generateGamsString())
....
....
....

To see the generated GAMS statement for a certain symbol, getStatement function can be utilized.

from gamspy import Container, Set
m = Container()
i = Set(m, "i", records=['i1', 'i2'])
print(i.getStatement())

The code snippet above prints the GAMS statement for the symbol i:

'Set i(*);'